打卡（7）

气死我了，又一次没保存？
https://leetcode-cn.com/problems/range-sum-query-2d-immutable/

class NumMatrix {
    
    
public:
    vector<vector<int>> sum;
    NumMatrix(vector<vector<int>>& matrix) {
    
    
        int n=matrix.size();
        if(n>0){
    
    //因为没考虑特殊情况造成了runtime error
            int m=matrix[0].size();
            sum.resize(n+1,vector<int>(m+1));//这样resize是最简单的方式
            for(int i=0;i<n;i++){
    
    
                for(int j=0;j<m;j++){
    
    
                    sum[i+1][j+1]=sum[i+1][j]+matrix[i][j]+sum[i][j+1]-sum[i][j];
                }
            }
        }
    }
    int sumRegion(int row1, int col1, int row2, int col2) {
    
    
        return sum[row2+1][col2+1]+sum[row1][col1]-sum[row1][col2+1]-sum[row2+1][col1];//这个公式需要记住
    }
};

/**
 * Your NumMatrix object will be instantiated and called as such:
 * NumMatrix* obj = new NumMatrix(matrix);
 * int param_1 = obj->sumRegion(row1,col1,row2,col2);
 */

salute！