304. Range Sum Query 2D
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304.Range Sum Query 2D - Immutable
二维区域和检索
题目:
给定一个二维数组matrix[][],以及两个坐标点(row1,col1)(row2,col2),求由两个坐标点围成的矩形区域中的元素之和。
解题思路:
利用动态规划的思想。定义dp[i][j]为区间(0,0)-(i,j)这个矩形区域中的所有元素之和。
dp[n+1][n+1]
dp[i][j]=dp[i][j-1]+dp[i-1][j]-dp[i-1][j-1]+matrix[i-1][j-1]
result=dp[row2+1][col2+1]-dp[row1][col2+1]-dp[row2+1][col1]+dp[row1][col1]
代码:
class NumMatrix{public NumMatrix(int[][] matrix){if(matrix==null || matrix.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){return dp[row2+1][col2+1]-dp[row1][col2+1]-dp[row2+1][col1]+dp[row1][col1];}private int [][]dp;}
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- 304. Range Sum Query 2D - Immutable
- 304. Range Sum Query 2D - Immutable
- 304. Range Sum Query 2D - Immutable
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- 304. Range Sum Query 2D - Immutable
- 304. Range Sum Query 2D - Immutable
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