力扣304.二维区域和检索

• 二维前缀和的简单应用

•   class NumMatrix {
        vector<vector<int>> sum;
    public:
        NumMatrix(vector<vector<int>>& matrix) {
            int m = matrix.size(),n = matrix[0].size();
            sum.resize(m+1,vector<int>(n+1));  
            for(int i=0;i<m;i++)
                for(int j=0;j<n;j++)
                    sum[i+1][j+1] = sum[i+1][j] + sum[i][j+1] - sum[i][j] + matrix[i][j];
        }
    
        
        int sumRegion(int r1, int c1, int r2, int c2) {
            return sum[r2+1][c2+1] - sum[r1][c2+1] - sum[r2+1][c1] + sum[r1][c1];
        }
    };