(DP 线性DP 递推) leetcode 63. Unique Paths II

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).

Now consider if some obstacles are added to the grids. How many unique paths would there be?

An obstacle and empty space is marked as 1 and 0 respectively in the grid.

Note: m and n will be at most 100.

Example 1:

Input:
[
  [0,0,0],
  [0,1,0],
  [0,0,0]
]
Output: 2
Explanation:
There is one obstacle in the middle of the 3x3 grid above.
There are two ways to reach the bottom-right corner:
1. Right -> Right -> Down -> Down
2. Down -> Down -> Right -> Right

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这个是leetcode 62. Unique Paths的拓展题。其中不同点就是这个题有障碍。所以，面对这个障碍是要避开。

C++代码：

class Solution {
public:
    int uniquePathsWithObstacles(vector<vector<int>>& obstacleGrid) {
        if(obstacleGrid[0][0] == 1 || obstacleGrid.size() == 0 || obstacleGrid[0].size() == 0) return 0;   //如果出发的贷方有障碍，当然不能出发了。
        int m = obstacleGrid.size(),n = obstacleGrid[0].size();
        vector<vector<long> > dp(m+1,vector<long>(n+1,0));
        for(int i = 0; i < m; i++){
            if(obstacleGrid[i][0] == 1)
                break;
            dp[i][0] = 1;
        }
        for(int j = 0; j < n; j++){
            if(obstacleGrid[0][j] == 1)
                break;
            dp[0][j] = 1;
        }
        for(int i = 1; i < m; i++){
            for(int j = 1; j < n; j++){
                if(obstacleGrid[i][j] == 1)
                    continue;
                dp[i][j] = dp[i-1][j] + dp[i][j-1];
            }
        }
        return dp[m-1][n-1];
    }
};