Unique Paths

    //two dimensions dp    public int uniquePaths(int m, int n) {        // Start typing your Java solution below        // DO NOT write main() function        if(m == 0 || n == 0) return 0;        int[][] dp = new int[m][n];        for(int i = 0; i < m; i++) dp[i][0] = 1;        for(int j = 0; j < n; j++) dp[0][j] = 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];    }    //use rotate arrays to save space    public int uniquePaths(int m, int n) {        // Start typing your Java solution below        // DO NOT write main() function        if(m == 0 || n == 0) return 0;        int[][] dp = new int[2][n];        for(int i = 0; i < n; i++) dp[0][i] = 1;        int flag = 0;        for(int i = 1; i < m; i++) {            flag = i & 1;            dp[flag][0] = 1;            for(int j = 1; j < n; j++) {                dp[flag][j] = dp[flag ^ 1][j] + dp[flag][j - 1];            }        }        return dp[flag][n - 1];    }