LeetCode: Unique Binary Search Trees

有经验了，一上来就知道用dp， 一次过

class Solution {
public:
    int numTrees(int n) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        if (n < 2) return 1;
        vector<int> f(n+1, 0);
        f[0] = 1;
        f[1] = 1;
        for (int i = 2; i <= n; i++) {
            for (int j = 0; j < i; j++)
                f[i] += f[j]*f[i-1-j];
        }
        return f[n];
    }
};

 C#

public class Solution {
    public int NumTrees(int n) {
        if (n < 2) return 1;
        int[] f = new int[n+1];
        f[0] = f[1] = 1;
        for (int i = 2; i <= n; i++) {
            for (int j = 0; j < i; j++) {
                f[i] += f[j] * f[i-1-j];
            }
        }
        return f[n];
    }
}