LeetCode 304. 二维区域和检索 - 矩阵不可变（DP）

1. 题目

2. 解题

dp[i][j]数组表示 从左上角到i,j位置的所有和

sum[i 1][j 1] = sum[i 1][j] sum[i][j 1] matrix[i][j]-sum[i][j]

代码语言：javascript复制

class NumMatrix {
	vector<vector<int>> sum;
public:
    NumMatrix(vector<vector<int>>& matrix) {
        if(matrix.empty())
            return;
        int r = matrix.size(), c = matrix[0].size(), i, j;
        sum = vector<vector<int>> (r 1, vector<int>(c 1, 0));
        for(i = 0; i < r; i  )
        {
        	for(j = 0; j < c; j  )
        	{
        		sum[i 1][j 1] = sum[i 1][j] sum[i][j 1] matrix[i][j]-sum[i][j];
        	}
        }
    }
    
    int sumRegion(int row1, int col1, int row2, int col2) {
        if(sum.empty())
            return 0;
        return sum[row2 1][col2 1] - sum[row1][col2 1] - sum[row2 1][col1]   sum[row1][col1];
    }
};

or 按行dp

代码语言：javascript复制

class NumMatrix {
	vector<vector<int>> sumofrows;
public:
    NumMatrix(vector<vector<int>>& matrix) {
        if(matrix.empty())
            return;
        int r = matrix.size(), c = matrix[0].size(), i, j, sum = 0;
        vector<int> temp(c,0);
        for(i = 0; i < r; i  )
        {
        	sum = 0;
        	for(j = 0; j < c; j  )
        	{
        		sum  = matrix[i][j];
        		temp[j] = sum;
        	}
        	sumofrows.push_back(temp);
        }
    }
    
    int sumRegion(int row1, int col1, int row2, int col2) {
        if(sumofrows.empty())
            return 0;
        int i, j, sum = 0;
        if(col1 != 0)
            for(i = row1; i <= row2; i  )
            {
                sum  = sumofrows[i][col2]-sumofrows[i][col1-1];
            }
        else
            for(i = row1; i <= row2; i  )
            {
                sum  = sumofrows[i][col2];
            }
        return sum;
    }
};

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

dp 数组

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