Leetcode 304. Range Sum Query 2D - Immutable

/** * very similar with range sum in 1D array, need a 2D array to save the intermediate values * instantiate a row+1, col+1 array make lot easier* */ public class NumMatrix {    private int[][] dp;        public NumMatrix(int[][] matrix) {        if(   matrix           == null           || matrix.length    == 0           || matrix[0].length == 0   ){            return;           }                int row = matrix.length;        int col = matrix[0].length;        dp = new int[row+1][col+1];                for (int i=1; i<=row; i++)             for (int j=1; j<=col; 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) {        return dp[row2+1][col2+1] - dp[row1][col2+1] - dp[row2+1][col1] + dp[row1][col1];    }}// Your NumMatrix object will be instantiated and called as such:// NumMatrix numMatrix = new NumMatrix(matrix);// numMatrix.sumRegion(0, 1, 2, 3);// numMatrix.sumRegion(1, 2, 3, 4);