计算树中所有可能的路径，使得节点 X 不会出现在节点 Y 之前

给定一个由N 个节点组成的树，这些节点的值在[0, N – 1]和(N – 1) 条边的范围内，以及两个节点X和Y ，任务是找到树中可能路径的数量，使得节点X不会出现在路径中的节点Y之前。

例子：

Input: N = 5, A = 2, B = 0, Edges[][] = { {0, 1}, {1, 2}, {1, 3}, {0, 4} } 
Output: 18 
Explanation: 
Since (X, Y) and (Y, X) are being considered different, so the count of all possible paths connecting any two pairs of vertices = 2 * ⁵C₂ = 20. 
Out of these 20 pairs, those paths cannot be chosen, which consist of both nodes 2 and 0 as well as Node 2 appearing before Node 0. 
There are two such paths (colored as green) which are shown below:

 

So there are total 20 – 2 = 18 such paths.
 

Input: N = 9, X = 5, Y = 3, Edges[][] = { {0, 2}, {1, 2}, {2, 3}, {3, 4}, {4, 6}, {4, 5}, {5, 7}, {5, 8} } 
Output: 60 
Explanation: 
Since (X, Y) and (Y, X) are being considered different, so the count of all possible paths connecting any two pairs of vertices = N * (N – 1) = 9 * 8 = 72. 
On observing the diagram below, any path starting from a Node in the subtree of Node 5, denoted by black, connecting to the vertices passing through the Node 3, denoted by green, will always have 5 appearing before 3 in the path. 
 

Therefore, total number of possible paths = (Total Nodes grouped in black) * (Total Nodes grouped in green) = 3 * 4 = 12. 
Therefore, the final answer = 72 – 12 = 60  

方法：
这个想法是找到节点对的组合，在连接它们的路径中，节点 X 总是出现在节点 Y 之前。然后，从可能的节点对的总数中减去这些对的数量= ^N C ² 。将节点Y视为根节点。现在，首先遇到X然后遇到Y 的任何路径都从节点X的子树中的节点开始，并在节点Y的子树中的一个节点处结束，但不在节点W的子树中，其中W是节点Y位于这些路径中的X和Y之间。

因此，最终答案可以通过以下方式计算：

Count = N * (N – 1) – size_of_subtree(X) * (size_of_subtree(Y) – size_of_subtree(W))

如果将Y作为树的根。然后， size_of_subtree(Y) = N 。

Count = N * (N – 1) – size_of_subtree(X) * (N- size_of_subtree(W))

请按照以下步骤解决问题：

1. 初始化数组subtree_size [] 、 visited []和check_subtree []每个大小为N + 1 。将visited [] 的元素初始化为0 。
2. 以Y为根节点执行DFS遍历，为每个节点填充check_subtree[]和subtree_size[] 。 check_subtree[]检查当前节点的子树是否包含节点X。
3. 找到Y的子节点（比如节点 v） ，它位于从X到Y的路径中。初始化一个整数变量说差异。
4. 将（节点总数 – subtree_size[v] ）分配给差异。
5. 返回(N * (N – 1) ) – (subtree_size[A] * (difference))作为答案。

下面是上述方法的实现：

C++

// C++ Program to implement
// the above approach
#include 
#define int long long int
using namespace std;
 
// Maximum number of nodes
const int NN = 3e5;
 
// Vector to store the tree
vector G[NN + 1];
 
// Function to perform DFS Traversal
int dfs(int node, int A, int* subtree_size,
        int* visited, int* check_subtree)
{
    // Mark the node as visited
    visited[node] = true;
 
    // Initialize the subtree size
    // of each node as 1
    subtree_size[node] = 1;
 
    // If the node is same as A
    if (node == A) {
 
        // Mark check_subtree[node] as true
        check_subtree[node] = true;
    }
 
    // Otherwise
    else
        check_subtree[node] = false;
 
    // Iterate over the adjacent nodes
    for (int v : G[node]) {
 
        // If the adjacent node
        // is not visited
        if (!visited[v]) {
 
            // Update the size of the
            // subtree of current node
            subtree_size[node]
                += dfs(v, A, subtree_size,
                       visited, check_subtree);
 
            // Check if the subtree of
            // current node contains node A
            check_subtree[node] = check_subtree[node]
                                  | check_subtree[v];
        }
    }
 
    // Return size of subtree of node
    return subtree_size[node];
}
 
// Function to add edges to the tree
void addedge(int node1, int node2)
{
 
    G[node1].push_back(node2);
    G[node2].push_back(node1);
}
 
// Function to calculate the number of
// possible paths
int numberOfPairs(int N, int B, int A)
{
    // Stores the size of subtree
    // of each node
    int subtree_size[N + 1];
 
    // Stores which nodes are
    // visited
    int visited[N + 1];
 
    // Initialise all nodes as unvisited
    memset(visited, 0, sizeof(visited));
 
    // Stores if the subtree of
    // a node contains node A
    int check_subtree[N + 1];
 
    // DFS Call
    dfs(B, A, subtree_size,
        visited, check_subtree);
 
    // Stores the difference between
    // total number of nodes and
    // subtree size of an immediate
    // child of Y lies between the
    // path from A to B
    int difference;
 
    // Iterate over the adjacent nodes B
    for (int v : G[B]) {
 
        // If the node is in the path
        // from A to B
        if (check_subtree[v]) {
 
            // Calcualte the difference
            difference = N - subtree_size[v];
 
            break;
        }
    }
 
    // Return the final answer
    return (N * (N - 1))
           - difference * (subtree_size[A]);
}
 
// Driver Code
int32_t main()
{
    int N = 9;
 
    int X = 5, Y = 3;
 
    // Insert Edges
    addedge(0, 2);
    addedge(1, 2);
    addedge(2, 3);
    addedge(3, 4);
    addedge(4, 6);
    addedge(4, 5);
    addedge(5, 7);
    addedge(5, 8);
 
    cout << numberOfPairs(N, Y, X);
 
    return 0;
}

Java

// Java Program to implement
// the above approach
import java.util.*;
class GFG{
 
// Maximum number of nodes
static int NN = (int) 3e5;
 
// Vector to store the tree
static Vector []G = new Vector[NN + 1];
 
// Function to perform DFS Traversal
static int dfs(int node, int A, int[] subtree_size,
               int[] visited, int[] check_subtree)
{
    // Mark the node as visited
    visited[node] = 1;
 
    // Initialize the subtree size
    // of each node as 1
    subtree_size[node] = 1;
 
    // If the node is same as A
    if (node == A)
    {
 
        // Mark check_subtree[node] as true
        check_subtree[node] = 1;
    }
 
    // Otherwise
    else
        check_subtree[node] = 0;
 
    // Iterate over the adjacent nodes
    for (int v : G[node])
    {
 
        // If the adjacent node
        // is not visited
        if (visited[v] == 0)
        {
 
            // Update the size of the
            // subtree of current node
            subtree_size[node] += dfs(v, A, subtree_size,
                                      visited, check_subtree);
 
            // Check if the subtree of
            // current node contains node A
            check_subtree[node] = check_subtree[node] |
                                    check_subtree[v];
        }
    }
 
    // Return size of subtree of node
    return subtree_size[node];
}
 
// Function to add edges to the tree
static void addedge(int node1, int node2)
{
    G[node1].add(node2);
    G[node2].add(node1);
}
 
// Function to calculate the number of
// possible paths
static int numberOfPairs(int N, int B, int A)
{
    // Stores the size of subtree
    // of each node
    int []subtree_size = new int[N + 1];
 
    // Stores which nodes are
    // visited
    int []visited = new int[N + 1];
 
 
    // Stores if the subtree of
    // a node contains node A
    int []check_subtree = new int[N + 1];
 
    // DFS Call
    dfs(B, A, subtree_size,
        visited, check_subtree);
 
    // Stores the difference between
    // total number of nodes and
    // subtree size of an immediate
    // child of Y lies between the
    // path from A to B
    int difference = 0;
 
    // Iterate over the adjacent nodes B
    for (int v : G[B])
    {
 
        // If the node is in the path
        // from A to B
        if (check_subtree[v] > 0)
        {
 
            // Calcualte the difference
            difference = N - subtree_size[v];
 
            break;
        }
    }
 
    // Return the final answer
    return (N * (N - 1)) -
              difference * (subtree_size[A]);
}
 
// Driver Code
public static void main(String[] args)
{
    int N = 9;
 
    int X = 5, Y = 3;
     
    for (int i = 0; i < G.length; i++)
        G[i] = new Vector();
   
    // Insert Edges
    addedge(0, 2);
    addedge(1, 2);
    addedge(2, 3);
    addedge(3, 4);
    addedge(4, 6);
    addedge(4, 5);
    addedge(5, 7);
    addedge(5, 8);
 
    System.out.print(numberOfPairs(N, Y, X));
}
}
 
// This code is contributed by sapnasingh4991

Python3

# Python3 program to implement
# the above approach
 
# Maximum number of nodes
NN = int(3e5)
 
# Vector to store the tree
G = []
for i in range(NN + 1):
    G.append([])
 
# Function to perform DFS Traversal
def dfs(node, A, subtree_size,
        visited, check_subtree):
 
    # Mark the node as visited
    visited[node] = True
 
    # Initialize the subtree size
    # of each node as 1
    subtree_size[node] = 1
 
    # If the node is same as A
    if (node == A):
 
        # Mark check_subtree[node] as true
        check_subtree[node] = True
 
    # Otherwise
    else:
        check_subtree[node] = False
 
    # Iterate over the adjacent nodes
    for v in G[node]:
 
        # If the adjacent node
        # is not visited
        if (not visited[v]):
 
            # Update the size of the
            # subtree of current node
            subtree_size[node] += dfs(v, A,
                                      subtree_size,
                                      visited,
                                      check_subtree)
 
            # Check if the subtree of
            # current node contains node A
            check_subtree[node] = (check_subtree[node] |
                                   check_subtree[v])
 
    # Return size of subtree of node
    return subtree_size[node]
 
# Function to add edges to the tree
def addedge(node1, node2):
 
    G[node1] += [node2]
    G[node2] += [node1]
 
# Function to calculate the number of
# possible paths
def numberOfPairs(N, B, A):
 
    # Stores the size of subtree
    # of each node
    subtree_size = [0] * (N + 1)
 
    # Stores which nodes are
    # visited
    visited = [0] * (N + 1)
 
    # Stores if the subtree of
    # a node contains node A
    check_subtree = [0] * (N + 1)
 
    # DFS Call
    dfs(B, A, subtree_size,
        visited, check_subtree)
 
    # Stores the difference between
    # total number of nodes and
    # subtree size of an immediate
    # child of Y lies between the
    # path from A to B
    difference = 0
 
    # Iterate over the adjacent nodes B
    for v in G[B]:
 
        # If the node is in the path
        # from A to B
        if (check_subtree[v]):
 
            # Calcualte the difference
            difference = N - subtree_size[v]
            break
 
    # Return the final answer
    return ((N * (N - 1)) -
               difference * (subtree_size[A]))
 
# Driver Code
N = 9
X = 5
Y = 3
 
# Insert Edges
addedge(0, 2)
addedge(1, 2)
addedge(2, 3)
addedge(3, 4)
addedge(4, 6)
addedge(4, 5)
addedge(5, 7)
addedge(5, 8)
 
# Function call
print(numberOfPairs(N, Y, X))
 
# This code is contributed by Shivam Singh

C#

// C# Program to implement
// the above approach
using System;
using System.Collections.Generic;
 
class GFG{
 
// Maximum number of nodes
static int NN = (int) 3e5;
 
// List to store the tree
static List []G = new List[NN + 1];
 
// Function to perform DFS Traversal
static int dfs(int node, int A, int[] subtree_size,
               int[] visited, int[] check_subtree)
{
    // Mark the node as visited
    visited[node] = 1;
 
    // Initialize the subtree size
    // of each node as 1
    subtree_size[node] = 1;
 
    // If the node is same as A
    if (node == A)
    {
 
        // Mark check_subtree[node] as true
        check_subtree[node] = 1;
    }
 
    // Otherwise
    else
        check_subtree[node] = 0;
 
    // Iterate over the adjacent nodes
    foreach (int v in G[node])
    {
 
        // If the adjacent node
        // is not visited
        if (visited[v] == 0)
        {
 
            // Update the size of the
            // subtree of current node
            subtree_size[node] += dfs(v, A, subtree_size,
                                      visited, check_subtree);
 
            // Check if the subtree of
            // current node contains node A
            check_subtree[node] = check_subtree[node] |
                                    check_subtree[v];
        }
    }
 
    // Return size of subtree of node
    return subtree_size[node];
}
 
// Function to add edges to the tree
static void addedge(int node1, int node2)
{
    G[node1].Add(node2);
    G[node2].Add(node1);
}
 
// Function to calculate the number of
// possible paths
static int numberOfPairs(int N, int B, int A)
{
    // Stores the size of subtree
    // of each node
    int []subtree_size = new int[N + 1];
 
    // Stores which nodes are
    // visited
    int []visited = new int[N + 1];
 
 
    // Stores if the subtree of
    // a node contains node A
    int []check_subtree = new int[N + 1];
 
    // DFS Call
    dfs(B, A, subtree_size,
        visited, check_subtree);
 
    // Stores the difference between
    // total number of nodes and
    // subtree size of an immediate
    // child of Y lies between the
    // path from A to B
    int difference = 0;
 
    // Iterate over the adjacent nodes B
    foreach (int v in G[B])
    {
 
        // If the node is in the path
        // from A to B
        if (check_subtree[v] > 0)
        {
 
            // Calcualte the difference
            difference = N - subtree_size[v];
 
            break;
        }
    }
 
    // Return the readonly answer
    return (N * (N - 1)) -
              difference * (subtree_size[A]);
}
 
// Driver Code
public static void Main(String[] args)
{
    int N = 9;
 
    int X = 5, Y = 3;
     
    for (int i = 0; i < G.Length; i++)
        G[i] = new List();
   
    // Insert Edges
    addedge(0, 2);
    addedge(1, 2);
    addedge(2, 3);
    addedge(3, 4);
    addedge(4, 6);
    addedge(4, 5);
    addedge(5, 7);
    addedge(5, 8);
 
    Console.Write(numberOfPairs(N, Y, X));
}
}
 
 
// This code is contributed by sapnasingh4991

Javascript

    // Javascript Program to implement the above approach
     
    // Maximum number of nodes
    let NN = 3e5;
 
    // Vector to store the tree
    let G = new Array(NN + 1);
 
    // Function to perform DFS Traversal
    function dfs(node, A, subtree_size, visited, check_subtree)
    {
        // Mark the node as visited
        visited[node] = 1;
 
        // Initialize the subtree size
        // of each node as 1
        subtree_size[node] = 1;
 
        // If the node is same as A
        if (node == A)
        {
 
            // Mark check_subtree[node] as true
            check_subtree[node] = 1;
        }
 
        // Otherwise
        else
            check_subtree[node] = 0;
 
        // Iterate over the adjacent nodes
        for (let v = 0; v < G[node].length; v++)
        {
 
            // If the adjacent node
            // is not visited
            if (visited[G[node][v]] == 0)
            {
 
                // Update the size of the
                // subtree of current node
                subtree_size[node] += dfs(G[node][v], A, subtree_size,
                                          visited, check_subtree);
 
                // Check if the subtree of
                // current node contains node A
                check_subtree[node] = check_subtree[node] |
                                        check_subtree[G[node][v]];
            }
        }
 
        // Return size of subtree of node
        return subtree_size[node];
    }
 
    // Function to add edges to the tree
    function addedge(node1, node2)
    {
        G[node1].push(node2);
        G[node2].push(node1);
    }
 
    // Function to calculate the number of
    // possible paths
    function numberOfPairs(N, B, A)
    {
        // Stores the size of subtree
        // of each node
        let subtree_size = new Array(N + 1);
        subtree_size.fill(0);
 
        // Stores which nodes are
        // visited
        let visited = new Array(N + 1);
        visited.fill(0);
 
 
        // Stores if the subtree of
        // a node contains node A
        let check_subtree = new Array(N + 1);
        check_subtree.fill(0);
 
        // DFS Call
        dfs(B, A, subtree_size, visited, check_subtree);
 
        // Stores the difference between
        // total number of nodes and
        // subtree size of an immediate
        // child of Y lies between the
        // path from A to B
        let difference = 0;
 
        // Iterate over the adjacent nodes B
        for (let v = 0; v < G[B].length; v++)
        {
 
            // If the node is in the path
            // from A to B
            if (check_subtree[G[B][v]] > 0)
            {
 
                // Calcualte the difference
                difference = N - subtree_size[G[B][v]];
 
                break;
            }
        }
 
        // Return the final answer
        return (N * (N - 1)) -
                  difference * (subtree_size[A]);
    }
     
    let N = 9;
   
    let X = 5, Y = 3;
       
    for (let i = 0; i < G.length; i++)
        G[i] = [];
     
    // Insert Edges
    addedge(0, 2);
    addedge(1, 2);
    addedge(2, 3);
    addedge(3, 4);
    addedge(4, 6);
    addedge(4, 5);
    addedge(5, 7);
    addedge(5, 8);
   
    document.write(numberOfPairs(N, Y, X));
 

输出：

60

时间复杂度： O(N)
辅助空间： O(N)

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