304.二维区域和检索-矩阵不可变.cpp

/*
 * @lc app=leetcode.cn id=304 lang=cpp
 *
 * [304] 二维区域和检索 - 矩阵不可变
 */

// @lc code=start
class NumMatrix {
    private: vector<vector<int>> preSum;
public:
    NumMatrix(vector<vector<int>>& matrix) {
        int m = matrix.size();
        int n = matrix[0].size();
        if (!m || !n) return;   // 需要注意我们要排除特殊情况。
        // 接下来就可以构造前缀和矩阵
        preSum = vector<vector<int>> (m+1, vector<int> (n+1));
        for (int i = 1; i <= m; i++)
            for (int j = 1; j <= n; j++) 
                preSum[i][j] = preSum[i-1][j] + preSum[i][j-1] + matrix[i-1][j-1] - preSum[i-1][j-1];
    }
    
    int sumRegion(int row1, int col1, int row2, int col2) {
        return preSum[row2+1][col2+1] - preSum[row1][col2+1] - preSum[row2+1][col1] + preSum[row1][col1];
    }
};

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