[LeetCode]304. Range Sum Query 2D - Immutable

Problem Description

[]https://leetcode.com/problems/range-sum-query-2d-immutable/]

Given a 2D matrix matrix, find the sum of the elements inside the rectangle defined by its upper left corner (row1, col1) and lower right corner (row2, col2).

思路

DP ,越简单的题，越是容易WA。。。。

Code

package q304;

public class NumMatrix {

private int[][] sum;
private int m = 0, n = 0;

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

m = matrix.length;
n = matrix[0].length;

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

}

public int sumRegion(int row1, int col1, int row2, int col2) {
if (m == 0 || n == 0)
return -1;
if (row1 >= m || row1 < 0)
return -1;
if (row2 >= m || row2 < 0)
return -1;
if (col1 >= n || row2 < 0)
return -1;
if (col2 >= n || row2 < 0)
return -1;
return sum[row2 + 1][col2 + 1] - sum[row2 + 1][col1]
- sum[row1][col2 + 1] + sum[row1][col1];
}
}