63. 不同路径 II

动态规划：

• 状态定义：dp[i][j]表示走到第i行第j列的路径数量，dp[m-1][n-1]即为答案。
• 状态转移：dp[i][j] = dp[i - 1][j] + dp[i][j - 1]
• 初始化：dp[0][0] = 1（当左上角位置不是障碍物的情况）

class Solution {
    public int uniquePathsWithObstacles(int[][] obstacleGrid) {
        int ans = 0;
        if (obstacleGrid[0][0] == 1) return ans;
        int m = obstacleGrid.length, n = obstacleGrid[0].length;
        int[][] dp = new int[m][n];
        dp[0][0] = 1;
        for (int i = 1; i < m; ++i) {
            if (obstacleGrid[i][0] == 1) dp[i][0] = 0;
            else dp[i][0] = dp[i - 1][0];
        }
        for (int j = 1; j < n; ++j) {
            if (obstacleGrid[0][j] == 1) dp[0][j] = 0;
            else dp[0][j] = dp[0][j - 1];
        }
        for (int i = 1; i < m; ++i) {
            for (int j = 1; j < n; ++j) {
                if (obstacleGrid[i][j] == 1) dp[i][j] = 0;
                else dp[i][j] = dp[i - 1][j] + dp[i][j - 1];
            }
        }
        return dp[m - 1][n - 1];
    }
}