https://oj.leetcode.com/problems/unique-paths/

Unique Paths.java

public class Solution {
    public int uniquePaths(int m, int n) {
        long ret = 1;
        int a = m - 1, b = n - 1;
        if ((a == 0) || (b == 0)) return 1;
        if (a > b){
            a = a ^ b;
            b = a ^ b;
            a = a ^ b;
        }
        for (int i = 1; i <= a; i++){
            ret = ret * (a + b + 1 - i) / i;
        }
        return (int)(ret);
    }
}

数学问题！