Leetcode #304. Range Sum Query 2D - Immutable

[yokostan]

Well, use DP instead of Brute Force

class NumMatrix {
    int[][] dp;

    public NumMatrix(int[][] matrix) {
        if (matrix == null || matrix.length == 0 || matrix[0].length == 0) return ;
        int m = matrix.length;
        int n = matrix[0].length;

        dp = new int[m + 1][n + 1];
        for (int i = 1; i <= m; i++) {
            for (int j = 1; j <= n; j++) {
                dp[i][j] = dp[i - 1][j] + dp[i][j - 1] - dp[i - 1][j - 1] + matrix[i - 1][j - 1];
            }
        }
    }

    public int sumRegion(int row1, int col1, int row2, int col2) {
        int iMin = Math.min(row1, row2);
        int iMax = Math.max(row1, row2);
        int jMin = Math.min(col1, col2);
        int jMax = Math.max(col1, col2);

        return dp[iMax + 1][jMax + 1] - dp[iMin][jMax + 1] - dp[iMax + 1][jMin] + dp[iMin][jMin];
    }
}

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