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1572. Matrix Diagonal Sum.md

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1572. Matrix Diagonal Sum

the first one in Biweekly Contest 34.

Difficulty : Easy

Related Topics : Array

Given a square matrix mat, return the sum of the matrix diagonals.

Only include the sum of all the elements on the primary diagonal and all the elements on the secondary diagonal that are not part of the primary diagonal.

Example 1:

1

Input: mat = [[1,2,3],
              [4,5,6],
              [7,8,9]]
Output: 25
Explanation: Diagonals sum: 1 + 5 + 9 + 3 + 7 = 25
Notice that element mat[1][1] = 5 is counted only once.

Example 2:

Input: mat = [[1,1,1,1],
              [1,1,1,1],
              [1,1,1,1],
              [1,1,1,1]]
Output: 8

Example 3:

Input: mat = [[5]]
Output: 5

Constraints:

  • n == mat.length == mat[i].length
  • 1 <= n <= 100
  • 1 <= mat[i][j] <= 100

Solution

  • mine
    • Java
      • Runtime: 0 ms, faster than 100.00%, Memory Usage: 39.1 MB, less than 100.00% of Java online submissions 
        //O(N)time
//O(1)space
public int diagonalSum(int[][] mat) {
    int r = mat.length;
    if(r == 1) return mat[0][0];
    int s = 0, e = r - 1;
    int x = 0;
    int res = 0;
    while(x < r){
        res += mat[x][s] + mat[x][e];
        x++;
        s++;
        e--;
    }
    if(r % 2 != 0){
        res -= mat[r / 2][r / 2];
    }
    return res;
}