(动态规划)不同的二叉搜索树

class Solution {
public:
    //建立动态规划方程
    //dp[i] = S(dp[j] * dp[i-j-1]) , 0 <= j < i
    int numTrees(int n) {
        vector<int> dp(n + 1, 0);
        dp[0] = 1;
        dp[1] = 1;
        for(int i = 2; i <= n; i ++){
            int targets = 0;
            for(int j = 0; j < i; j ++)
                targets += dp[j] * dp[i - j - 1];
            dp[i] = targets;
        }
        return dp[n];
    }
};