1572. Matrix Diagonal Sum 

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class Solution {
    public int diagonalSum(int[][] mat) {
        int n = mat.length;
        int res = 0;

        for (int i = 0; i < n; i++) {
            for (int j = 0; j < n; j++) {
                if (i == j || i + j == n - 1) {
                    res += mat[i][j];
                }
            }
        }

        return res;
    }
}
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class Solution {
    public int diagonalSum(int[][] mat) {
        int n = mat.length, mid = n / 2;
        int res = 0;

        for (int i = 0; i < n; i++) {
            res += mat[i][i] + mat[i][n - 1 - i];
        }

        return res - mat[mid][mid] * (n & 1);
    }
}

References

1572. Matrix Diagonal Sum