Problem
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).
https://leetcode.com/static/i...
The above rectangle (with the red border) is defined by (row1, col1) = (2, 1) and (row2, col2) = (4, 3), which contains sum = 8.
Example:
Given matrix = [ [3, 0, 1, 4, 2], [5, 6, 3, 2, 1], [1, 2, 0, 1, 5], [4, 1, 0, 1, 7], [1, 0, 3, 0, 5] ] sumRegion(2, 1, 4, 3) -> 8 sumRegion(1, 1, 2, 2) -> 11 sumRegion(1, 2, 2, 4) -> 12
Note:
You may assume that the matrix does not change.
There are many calls to sumRegion function.
You may assume that row1 ≤ row2 and col1 ≤ col2.
class NumMatrix { int[][] sum; public NumMatrix(int[][] matrix) { if (matrix == null || matrix.length == 0 || matrix[0].length == 0) { return; } int m = matrix.length, n = matrix[0].length; this.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) { return sum[row2+1][col2+1]-sum[row2+1][col1]-sum[row1][col2+1]+sum[row1][col1]; } }
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