# 方矩阵的两个对角线中的行式公共元素 > 原文： [https://www.geeksforgeeks.org/row-wise-common-elements-two-diagonals-square-matrix/](https://www.geeksforgeeks.org/row-wise-common-elements-two-diagonals-square-matrix/) 给定一个方阵，找出在同一行中以及在主和次对角线上都相同的数字计数。 **示例**： ``` Input : 1 2 1 4 5 2 0 5 1 Output : 2 Primary diagonal is 1 5 1 Secondary diagonal is 1 5 0 Two elements (1 and 5) match in two diagonals and same. Input : 1 0 0 0 1 0 0 0 1 Output : 1 Primary diagonal is 1 1 1 Secondary diagonal is 0 1 0 Only one element is same. ``` 我们可以在`O(n)`时间，`O(1)`空间和仅一个遍历中实现这一目标。 我们可以在主对角线的第 i 行找到当前元素作为 mat [i] [i]，在次对角线的第 i 个元素找到作为 mat [i] [n-i-1]。 ## C++ ```cpp // CPP program to find common elements in // two diagonals. #include <iostream> #define MAX 100 using namespace std; // Returns count of row wise same // elements in two diagonals of // mat[n][n] int countCommon(int mat[][MAX], int n) {     int res = 0;     for (int i=0;i<n;i++)         if (mat[i][i] == mat[i][n-i-1])              res++;     return res; } // Driver Code int main() {     int mat[][MAX] = {{1, 2, 3},                        {4, 5, 6},                       {7, 8, 9}};     cout << countCommon(mat, 3);     return 0; } ```