"Leetcode" Unique Binary Search Trees

Test instructions

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

Ideas:

n = 0 o'clock, empty tree, only one tree. n = 1 o'clock, only one possibility, also 1.

n >= 2 o'clock, the 12....N, respectively, I is the root node, then the left has I-1 nodes, the right side of the n-i-1 node, so

F[n] + = f[k-1]*f[n-k-1], k =,...., n-1

Code:

C++:

Class Solution {public:    int numtrees (int n) {        int *cnt = new Int[n+1];        Memset (cnt,0, (n+1) *sizeof (int));        Cnt[0] = 1;        CNT[1] = 1;        for (int i = 2;i <= n;i++) for            (int j = 0;j < I;++j)                Cnt[i] + = cnt[j]*cnt[i-j-1];        int sum = Cnt[n];        delete []cnt;        return sum;    }};

Python:

Class solution:    # @return An integer    def numtrees (self, N):        f = [0 for x in range (0,n+1)]        f[0] = 1
   F[1] = 1 for                i in range (2,n+1): for            J in Range (0,i):                f[i] + = f[j]*f[i-j-1]                return f[n]

"Leetcode" Unique Binary Search Trees