Leetcode oj:unique binary search Trees (unique binary searching tree)

Given N, how many structurally unique BST's (binary search trees) that store values 1 ... n?

For example,
Given N = 3, there is a total of 5 unique BST ' s.

   1         3     3      2      1    \//      /\           3     2     1      1   3      2    /     /       \                    2     1         2                 3

The first is to use recursion to solve, but see the label is DP problem, think about, the number of BST, 0 ~ k-1 can be divided into two intervals, and then can be generated BST, so K of the number of BST is equal to K left and right can be divided into BST the sum of the flight, the amount, a bit around the mouth amount, The code is a little clearer, look at the code:

1 classSolution {2  Public:3     intNumtrees (intN) {4vector<int>ret;5         if(n = =0)return 0;6Ret.reserve (n +1);7ret[0] =1;8          for(inti =1; I <= N; ++i) {9             if(I <3){TenRet[i] =i; One                 Continue; A             } -              for(intj =0; J < I; ++j) { -Ret[i] + = ret[j] * ret[i-j-1]; the             } -         } -         returnRet[n]; -     } +};

Leetcode oj:unique binary search Trees (unique binary searching tree)