leetcode 中等题(2)

50. Pow(x, n) (中等)

    double myPow(double x, int n) {
        double ans = 1;
        unsigned long long p;
        if (n < 0) {
            p = -n;
            x = 1 / x;
        } else {
            p = n;
        }
        while (p) {
            if (p & 1)
                ans *= x;
            x *= x;
            p >>= 1;
        }
        return ans;
    }

 96. Unique Binary Search Trees(很快)

1 class Solution {
2 public:
3     int numTrees(int n) {
4         long ans=1;
5         for(int i=n+1;i<=2*n;i++)
6             ans = i*ans/(i-n);
7         return ans/(n+1);
8     }
9 };

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