如果 c 等于 b 且 a 等于 d，则称两对 (a, b) 和 (c, d) 是对称的。例如，(10, 20) 和 (20, 10) 是对称的。给定一组对，找出其中的所有对称对。
可以假设所有对的第一个元素是不同的。
例子：
Input: arr[] = {{11, 20}, {30, 40}, {5, 10}, {40, 30}, {10, 5}}
Output: Following pairs have symmetric pairs
(30, 40)
(5, 10) 我们强烈建议您将浏览器最小化，然后自己先尝试一下。
一个简单的解决方案是遍历每一对，并检查其他每一对是否对称。该解决方案需要 O(n 2 ) 时间。
更好的解决方案是使用排序。按第一个元素对所有对进行排序。对于每一对，对给定数组中的第二个元素进行二分搜索，即检查该对的第二个元素是否作为数组中的第一个元素存在。如果找到，则将 pair 的第一个元素与第二个元素进行比较。该解决方案的时间复杂度为 O(nLogn)。
一个有效的解决方案是使用哈希。 pair 的第一个元素用作键，第二个元素用作值。这个想法是一个一个地遍历所有对。对于每一对，检查它的第二个元素是否在哈希表中。如果是，则将第一个元素与哈希表的匹配条目的值进行比较。如果值和第一个元素匹配，那么我们找到了对称对。否则，插入第一个元素作为键，第二个元素作为值。
下面是这个想法的实现。
C++
#include
using namespace std;
// A C++ program to find all symmetric pairs in a given array of pairs
// Print all pairs that have a symmetric counterpart
void findSymPairs(int arr[][2], int row)
{
// Creates an empty hashMap hM
unordered_map hM;
// Traverse through the given array
for (int i = 0; i < row; i++)
{
// First and second elements of current pair
int first = arr[i][0];
int sec = arr[i][1];
// If found and value in hash matches with first
// element of this pair, we found symmetry
if (hM.find(sec) != hM.end() && hM[sec] == first)
cout << "(" << sec << ", " << first << ")" < Java
// A Java program to find all symmetric pairs in a given array of pairs
import java.util.HashMap;
class SymmetricPairs {
// Print all pairs that have a symmetric counterpart
static void findSymPairs(int arr[][])
{
// Creates an empty hashMap hM
HashMap hM = new HashMap();
// Traverse through the given array
for (int i = 0; i < arr.length; i++)
{
// First and second elements of current pair
int first = arr[i][0];
int sec = arr[i][1];
// Look for second element of this pair in hash
Integer val = hM.get(sec);
// If found and value in hash matches with first
// element of this pair, we found symmetry
if (val != null && val == first)
System.out.println("(" + sec + ", " + first + ")");
else // Else put sec element of this pair in hash
hM.put(first, sec);
}
}
// Driver method
public static void main(String arg[])
{
int arr[][] = new int[5][2];
arr[0][0] = 11; arr[0][1] = 20;
arr[1][0] = 30; arr[1][1] = 40;
arr[2][0] = 5; arr[2][1] = 10;
arr[3][0] = 40; arr[3][1] = 30;
arr[4][0] = 10; arr[4][1] = 5;
findSymPairs(arr);
}
} Python3
# A Python3 program to find all symmetric
# pairs in a given array of pairs.
# Print all pairs that have
# a symmetric counterpart
def findSymPairs(arr, row):
# Creates an empty hashMap hM
hM = dict()
# Traverse through the given array
for i in range(row):
# First and second elements
# of current pair
first = arr[i][0]
sec = arr[i][1]
# If found and value in hash matches with first
# element of this pair, we found symmetry
if (sec in hM.keys() and hM[sec] == first):
print("(", sec,",", first, ")")
else: # Else put sec element of
# this pair in hash
hM[first] = sec
# Driver Code
if __name__ == '__main__':
arr = [[0 for i in range(2)]
for i in range(5)]
arr[0][0], arr[0][1] = 11, 20
arr[1][0], arr[1][1] = 30, 40
arr[2][0], arr[2][1] = 5, 10
arr[3][0], arr[3][1] = 40, 30
arr[4][0], arr[4][1] = 10, 5
findSymPairs(arr, 5)
# This code is contributed by Mohit KumarC#
// C# program to find all symmetric
// pairs in a given array of pairs
using System;
using System.Collections.Generic;
public class SymmetricPairs
{
// Print all pairs that have a symmetric counterpart
static void findSymPairs(int [,]arr)
{
// Creates an empty hashMap hM
Dictionary hM = new Dictionary();
int val = 0;
// Traverse through the given array
for (int i = 0; i < arr.GetLength(0); i++)
{
// First and second elements of current pair
int first = arr[i, 0];
int sec = arr[i, 1];
// Look for second element of this pair in hash
if(hM.ContainsKey(sec))
val = hM[sec];
// If found and value in hash matches with first
// element of this pair, we found symmetry
if (val != 0 && val == first)
Console.WriteLine("(" + sec + ", " + first + ")");
else // Else put sec element of this pair in hash
hM.Add(first, sec);
}
}
// Driver code
public static void Main(String []arg)
{
int [,]arr = new int[5, 2];
arr[0, 0] = 11; arr[0, 1] = 20;
arr[1, 0] = 30; arr[1, 1] = 40;
arr[2, 0] = 5; arr[2, 1] = 10;
arr[3, 0] = 40; arr[3, 1] = 30;
arr[4, 0] = 10; arr[4, 1] = 5;
findSymPairs(arr);
}
}
// This code has been contributed by 29AjayKumar Javascript
输出：
Following pairs have symmetric pairs
(30, 40)
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