1、题目
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?
Note: m and n will be at most 100.
Example 1:
Input: m = 3, n = 2 Output: 3 Explanation: From the top-left corner, there are a total of 3 ways to reach the bottom-right corner: 1. Right -> Right -> Down 2. Right -> Down -> Right 3. Down -> Right -> Right
Example 2:
Input: m = 7, n = 3 Output: 28
2、分析
及其经典的动态规划题。这类题目做的太多了,思路十分明确。机器人只能往右边和下边走。那么机器人到达一个位置,只能是从该位置的上面或者左边过来。假如到达该位置的上面有a种方法,到达该位置的左面有b种方法,那么到达该位置就有a+b种方法。递推就出来了。用dp数组来表示。dp[i][j]为到达第i行第j列的位置的方法数。一开始的时候第一行和第一列要初始化为1.然后递推就是dp[i][j] = dp[i-1][j]+dp[i][j-1]。注意只有一个格子的特殊情况即可。
3、代码
class Solution {
public:
int uniquePaths(int m, int n) {
int dp[m][n] = {0};
dp[0][0]=1;
for(int i=1;i<m;i++) dp[i][0] = 1;
for(int i=1;i<n;i++) dp[0][i] = 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];
}
};