96. Unique Binary Search Trees (Tree; DFS)

標籤：

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

For example,
Given n = 3, there are a total of 5 unique BST‘s.

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

  

class Solution {public:    int numTrees(int n) {        return dfs(1,n);    }       int dfs(int start, int end)    {        int leftNum = 0;        int rightNum = 0;        int num = 0;         for ( int i = start; i <= end; i++) //枚舉左、右子數節點分配的所有情況        {            if(i!=start) leftNum = dfs(start,i-1); //左子樹            else leftNum = 1;            if(i!=end) rightNum = dfs(i+1,end); //右子樹            else rightNum = 1;            num += leftNum*rightNum;        }               return num;    }};

 

96. Unique Binary Search Trees (Tree; DFS)