Type：medium
A robot is located at the top-left corner of a m x n grid (marked 'Start' in the diagram below).
The robot can only move either down or right at any point in time. The robot is trying to reach the bottom-right corner of the grid (marked 'Finish' in the diagram below).
How many possible unique paths are there?
给定一幅格子，从左上角格子走起，只能向下或向右走，问走到右下角格子有几种方法？
dp题。设原点为(0， 0)，则dp[i][j] = dp[i-1][j] + dp[i][j-1]。
class Solution {
public:
int uniquePaths(int m, int n) {
vector<vector<int>> dp(m, vector<int>(n, 0));
for(int i=0; i<n; i++) dp[0][i] = 1;
for(int i=1; i<m; i++) dp[i][0] = 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];
}
}
return dp[m-1][n-1];
}
};
