UniquePathsII.java

package Leetcode;

/**
 * @author kalpak
 *
 * 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 space is marked as 1 and 0 respectively in the grid.
 *
 *
 * Example 1:
 * Input: obstacleGrid = [[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
 *
 *
 * Example 2:
 * Input: obstacleGrid = [[0,1],[0,0]]
 * Output: 1
 *
 *
 * Constraints:
 *
 * m == obstacleGrid.length
 * n == obstacleGrid[i].length
 * 1 <= m, n <= 100
 * obstacleGrid[i][j] is 0 or 1.
 */

public class UniquePathsII {
    public static int uniquePathsWithObstacles(int[][] obstacleGrid) {
        if(obstacleGrid == null || obstacleGrid.length == 0 || obstacleGrid[0].length == 0)
            return 0;

        int m = obstacleGrid.length;
        int n = obstacleGrid[0].length;

        int[][] dp = new int [m][n];

        // First Column
        for(int i = 0; i < m; i++) {
            if(obstacleGrid[i][0] != 1)
                dp[i][0] = 1;
            else
                break;                      // Important: Since there is an obstacle, cant proceed further
        }

        // First Row
        for(int i = 0; i < n; i++) {
            if(obstacleGrid[0][i] != 1)
                dp[0][i] = 1;
            else
                break;                      // Important: Since there is an obstacle, cant proceed further
        }


        for(int i = 1; i < m; i++) {
            for(int j = 1; j < n; j++) {
                if(obstacleGrid[i][j] != 1) {
                    dp[i][j] = dp[i-1][j] + dp[i][j-1];
                }
            }
        }

        return dp[m-1][n-1];
    }

    public static void main(String[] args) {
        int[][] obstacleGrid = new int[][]{{0,0,0},{0,1,0},{0,0,0}};
        System.out.println(uniquePathsWithObstacles(obstacleGrid));
    }
}