LeetCode 230. Kth Smallest Element in a BST
2017-06-28 16:10
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题目
Given a binary search tree, write a function kthSmallest to find the kth smallest element in it.Note:
You may assume k is always valid, 1 ≤ k ≤ BST’s total elements.
思路
给定一个二叉搜索树,实现一个函数kthSmallest,找到这个BST中第k小的数。根据BST的性质:
左子树中的所有节点都小于根节点,
右子树中的所有节点都大于根节点
可以想到利用中序遍历(左->中->右)来遍历BST后得到的序列是一个递增的序列,然后返回这个序列中第k个数即可。
用递归实现中序遍历
需要注意的是,递归函数第二个参数是待返回容器,函数中会修改这个值,所以要用引用来作为参数。
代码
/** * Definition for a binary tree node. * struct TreeNode { * int val; * TreeNode *left; * TreeNode *right; * TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */ class Solution { public: int kthSmallest(TreeNode* root, int k) { if(root == NULL) return -1; vector<int> res; res = inorder(root,res); return res[k-1]; } vector<int> inorder(TreeNode* root,vector<int>& res)//引用调用 { if(root == NULL) return res; if(root->left != NULL) res=inorder(root->left,res); res.push_back(root->val); if(root->right != NULL) res=inorder(root->right,res); return res; } };
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