LeetCode304. Range Sum Query 2D - Immutable

题目链接：

题目描述：

题目分析：

sums[i][j]=sums[i-1][j]+sums[i][j-1]-sums[i-1][j-1]+matrix[i][j];

代码：

class NumMatrix {
public:
NumMatrix(vector<vector<int>> &matrix) {
int row=matrix.size();
if(row==0){
return;
}
int col=matrix[0].size();
sums=vector<vector<int>>(row,vector<int>(col,0));
sums[0][0]=matrix[0][0];
for(int i=1;i<row;i++){
sums[i][0]=sums[i-1][0]+matrix[i][0];
}
for(int i=1;i<col;i++){
sums[0][i]=sums[0][i-1]+matrix[0][i];
}
for(int i=1;i<row;i++){
for(int j=1;j<col;j++){
sums[i][j]=sums[i-1][j]+sums[i][j-1]-sums[i-1][j-1]+matrix[i][j];
}
}
}

int sumRegion(int row1, int col1, int row2, int col2) {
if(row1>row2 || col1>col2){
return 0;
}
if(row1==0 && col1==0){
return sums[row2][col2];
}
else if(row1==0){
return sums[row2][col2]-sums[row2][col1-1];
}
else if(col1==0){
return sums[row2][col2]-sums[row1-1][col2];
}
else{
return sums[row2][col2]-sums[row2][col1-1]-sums[row1-1][col2]+sums[row1-1][col1-1];
}
}
private:
vector<vector<int>> sums;
};