给定一个初始数组A[]和最终数组B[]两个大小为N 的数组都包含范围[1, N]中的整数，其中A[]表示插入元素的顺序， B[]表示在它们被删除，任务是找到B[] 中位于索引处的元素数量，该索引早于其在A[] 中的相应位置。
例子：
Input: A[ ] = {1, 2, 4, 3}, B[ ] = {2, 3, 4, 1}
Output: 3
Explanation:
The element 2 was inserted after 1, but removed before 1.
The element 3 was inserted after 4, but removed before 4.
The element 4 was inserted after 1, but removed before 1.
Hence, total 3 elements have moved ahead.
Input: A[ ] = {1, 2, 3, 4} B[ ] = {1, 2, 4, 3}
Output: 1
Explanation:
The element 4 was inserted after 3, but removed before 3.
Hence, only 1 element has moved ahead.
朴素的方法：解决这个问题最简单的方法是将B[]中每个元素的位置与A[]中所有其他元素的位置进行比较，并检查它是否插入在A[] 中的元素之后但在之前被移除乙[] 。如果是，则增加计数。最后，打印总计数。
时间复杂度： O(N 2 )
辅助空间： O(1)
高效的方法：解决这个问题的一个更好的方法是维护一个队列，将B[]的元素插入到这个队列中，并将所有元素插入到一个无序集合中。无序集合用于有效检查特定元素是否已被处理。遍历A[]并从队列中弹出元素，直到当前元素A[i]出现在队列顶部，同时不断增加计数并将出现在队列顶部的元素标记为已处理。最后，总计数将给出在B[] 中向前移动的元素数。
请按照以下步骤解决问题：
- 维护一个队列和无序集合，并将数组B[]的值存储在两者中。
- 现在，迭代数组A[] 。
- 如果当前元素不存在于无序集合中，则意味着它已经被删除。
- 初始化一个变量count 。如果在队列中找不到第i个元素，则从队列中移除队列中A[i]之前存在的所有元素，无序集合继续增加count 。
- 从队列和无序集合中移除A[i] 。
- 最后，打印 total count 。
下面是上述方法的实现：
C++
// C++ Program to implement
// the above appraoch
#include
using namespace std;
// Function returns maximum number
// of required elements
int maximumCount(int A[], int B[], int n)
{
queue q;
unordered_set s;
// Insert the elements of array B
// in the queue and set
for (int i = 0; i < n; i++) {
s.insert(B[i]);
q.push(B[i]);
}
// Stores the answer
int count = 0;
for (int i = 0; i < n; i++) {
// If A[i] is already processed
if (s.find(A[i]) == s.end())
continue;
// Until we find A[i] in the queue
while (!q.empty() && q.front() != A[i]) {
// Remove elements from the queue
s.erase(q.front());
q.pop();
// Increment the count
count++;
}
// Remove the current element A[i]
// from the queue and set.
if (A[i] == q.front()) {
q.pop();
s.erase(A[i]);
}
if (q.empty())
break;
}
// Return total count
cout << count << endl;
}
// Driver Code
int main()
{
int N = 4;
int A[] = { 1, 2, 3, 4 };
int B[] = { 1, 2, 4, 3 };
maximumCount(A, B, N);
return 0;
} Java
// Java program to implement
// the above appraoch
import java.util.*;
class GFG{
// Function returns maximum number
// of required elements
static void maximumCount(int A[], int B[], int n)
{
Queue q = new LinkedList<>();
HashSet s = new HashSet<>();
// Insert the elements of array B
// in the queue and set
for(int i = 0; i < n; i++)
{
s.add(B[i]);
q.add(B[i]);
}
// Stores the answer
int count = 0;
for(int i = 0; i < n; i++)
{
// If A[i] is already processed
if (!s.contains(A[i]))
continue;
// Until we find A[i] in the queue
while (!q.isEmpty() && q.peek() != A[i])
{
// Remove elements from the queue
s.remove(q.peek());
q.remove();
// Increment the count
count++;
}
// Remove the current element A[i]
// from the queue and set.
if (A[i] == q.peek())
{
q.remove();
s.remove(A[i]);
}
if (q.isEmpty())
break;
}
// Return total count
System.out.print(count + "\n");
}
// Driver Code
public static void main(String[] args)
{
int N = 4;
int A[] = { 1, 2, 3, 4 };
int B[] = { 1, 2, 4, 3 };
maximumCount(A, B, N);
}
}
// This code is contributed by princi singh Python3
# Python3 Program to implement
# the above appraoch
import queue
# Function returns maximum number
# of required elements
def maximumCount(A, B, n):
q = queue.Queue()
s = set()
# Insert the elements of
# array B in the queue
# and set
for i in range(n):
s.add(B[i])
q.put(B[i])
# Stores the answer
count = 0
for i in range(n):
# If A[i] is already
# processed
if (A[i] not in s):
continue
# Until we find A[i]
# in the queue
while (q.qsize() > 0 and
q.queue[0] != A[i]):
# Remove elements from
# the queue
s.remove(q.queue[0]);
q.get()
# Increment the count
count += 1
# Remove the current element A[i]
# from the queue and set.
if (A[i] == q.queue[0]):
q.get()
s.remove(A[i])
if (q.qsize() == 0):
break
# Return total count
print(count)
# Driver code
N = 4
A = [1, 2, 3, 4]
B = [1, 2, 4, 3]
maximumCount(A, B, N)
# This code is contributed by divyeshrabadiya07C#
// C# program to implement
// the above appraoch
using System;
using System.Collections.Generic;
class GFG{
// Function returns maximum number
// of required elements
static void maximumCount(int []A, int []B, int n)
{
Queue q = new Queue();
HashSet s = new HashSet();
// Insert the elements of array B
// in the queue and set
for(int i = 0; i < n; i++)
{
s.Add(B[i]);
q.Enqueue(B[i]);
}
// Stores the answer
int count = 0;
for(int i = 0; i < n; i++)
{
// If A[i] is already processed
if (!s.Contains(A[i]))
continue;
// Until we find A[i] in the queue
while (q.Count != 0 && q.Peek() != A[i])
{
// Remove elements from the queue
s.Remove(q.Peek());
q.Dequeue();
// Increment the count
count++;
}
// Remove the current element A[i]
// from the queue and set.
if (A[i] == q.Peek())
{
q.Dequeue();
s.Remove(A[i]);
}
if (q.Count == 0)
break;
}
// Return total count
Console.Write(count + "\n");
}
// Driver Code
public static void Main(String[] args)
{
int N = 4;
int []A = { 1, 2, 3, 4 };
int []B = { 1, 2, 4, 3 };
maximumCount(A, B, N);
}
}
// This code is contributed by princi singh Javascript
输出：
1时间复杂度： O(N)
辅助空间： O(N)
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