4.27 leetcode -27 unique-binary-search-trees
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题目描述
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
/ / \ \
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
这个题目,是一维的动态规划题目。每一个n的数目,可以由之前的数算出来
例如n = 5;我们可以假设 1作为root,那么左边没有数,右边有4个数,4个数可以组成情况数目,之前已经存起来了。
以此,2作为root,累加可得。
代码:
class Solution {public: int numTrees(int n) { int *p = (int*)malloc(sizeof(int) * (n+1)); p[0] = 1; if(n == 0) return 0; else { for(int i = 0;i < n;i ++) { p[i+1] = 0; for(int j = 0;j <= i;j ++) p[i+1] = p[i+1] + (p[j]*p[i-j]); } } return p[n]; }};
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