LeetCode Unique Binary Search Trees

Unique Binary Search Trees

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

动态规划法

本题关键：如何填表，如何得到计算公式.

需要很仔细画图，一步一步找出规律。

Catlan公式是可以计算这样的题目的，不过Catalan公式也不好理解，还是写个循环解决吧。

class Solution {
public:
	int numTrees(int n)
	{
		vector
  
    ta(n+1);
		ta[0] = 1; ta[1] = 1; ta[2] = 2;
		for (int i = 3; i <= n; i++)
		{
			int mid = (i-1)/2;

			for (int j = 0; j < mid; j++)
			{
				ta[i] += ta[j] * ta[i-j-1] *2;
			}

			if (i%2) ta[i] += ta[mid] * ta[mid];
			else ta[i] += ta[mid+1] * ta[mid] * 2;
		}
		return ta[n];
	}
};