问题：
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?
Above is a 3 x 7 grid. How many possible unique paths are there?
Note: m and n will be at most 100.
分析：
1、采用DFS，时间复杂度O(n^4)，大集合会超时；采用DFS+缓冲（即“备忘录”法）时间O(n^2)。
2、用动规，时间O(n^2)，空间O(n^2)；可以优化，后续补上。
3、还可以用数学公式，后续补上。
代码：
class Solution { public: int uniquePaths(int m, int n) { vector<vector<int>> p(m,vector<int>(n,1)); //边界 for(int i=0;i<m;i++) p[i][0]=1; for(int j=0;j<n;j++) p[0][j]=1; for(int i=1;i<m;i++) for(int j=1;j<n;j++){ //动规：自底向上 p[i][j]=p[i-1][j]+p[i][j-1]; } return p[m-1][n-1]; } };
