Add solution 0304

leetcode/0304.Range-Sum-Query-2D-Immutable/304. Range Sum Query 2D - Immutable.go

@@ -0,0 +1,31 @@
+package leetcode
+
+type NumMatrix struct {
+	cumsum [][]int
+}
+
+func Constructor(matrix [][]int) NumMatrix {
+	if len(matrix) == 0 {
+		return NumMatrix{nil}
+	}
+	cumsum := make([][]int, len(matrix)+1)
+	cumsum[0] = make([]int, len(matrix[0])+1)
+	for i := range matrix {
+		cumsum[i+1] = make([]int, len(matrix[i])+1)
+		for j := range matrix[i] {
+			cumsum[i+1][j+1] = matrix[i][j] + cumsum[i][j+1] + cumsum[i+1][j] - cumsum[i][j]
+		}
+	}
+	return NumMatrix{cumsum}
+}
+
+func (this *NumMatrix) SumRegion(row1 int, col1 int, row2 int, col2 int) int {
+	cumsum := this.cumsum
+	return cumsum[row2+1][col2+1] - cumsum[row1][col2+1] - cumsum[row2+1][col1] + cumsum[row1][col1]
+}
+
+/**
+ * Your NumMatrix object will be instantiated and called as such:
+ * obj := Constructor(matrix);
+ * param_1 := obj.SumRegion(row1,col1,row2,col2);
+ */