add: leetcode 1572 solution

leetcode/1572.Matrix-Diagonal-Sum/1572.Matrix Diagonal Sum.go

@@ -0,0 +1,16 @@
+package leetcode
+
+func diagonalSum(mat [][]int) int {
+	n := len(mat)
+	ans := 0
+	for pi := 0; pi < n; pi++ {
+		ans += mat[pi][pi]
+	}
+	for si, sj := n-1, 0; sj < n; si, sj = si-1, sj+1 {
+		ans += mat[si][sj]
+	}
+	if n%2 == 0 {
+		return ans
+	}
+	return ans - mat[n/2][n/2]
+}