LeetCode:: Unique Binary Search Trees[详细分析]

class Solution {
public:
    int numTrees(int n) {
       int sum;
       vector<int> a(n + 1);
       a[0] = 1;
       for (int i = 1; i <= n; i++){
           a[i] = 0;
           sum = 0;
           for (int j = 0; j < i; j++){
               sum += a[j] * a[i - 1 - j];
           }
           a[i] = sum;
       }
       return a[n];
    }
};