leetcode-230-Kth Smallest Element in a BST

标签：leetcode

                                    Kth Smallest Element in a BST

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.

Follow up:
What if the BST is modified (insert/delete operations) often and you need to find the kth smallest frequently? How would you optimize the kthSmallest routine?

找出一个二叉排序树中第k小的数。

中序遍历二叉排序树，将元素存在数组中，数组中的第K个数就是第K小的数。

递归

/**
 * 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:
    void zhongxu(TreeNode* root,int* a,int& i){
        if(!root) return;
        zhongxu(root->left,a,i);
        a[i++]=root->val;
        zhongxu(root->right,a,i);
    }
    int kthSmallest(TreeNode* root, int k) {
        int a[10000];
        int i=1;
        zhongxu(root,a,i);
        return a[k];
    }
};

/**
 * 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:
    void zhongxu(TreeNode* root,int& i,int& ans,int k){
        if(!root) return;
        zhongxu(root->left,i,ans,k);
        
        if(i++==k) ans=root->val;// i==k 时， 将答案存在来
        
        zhongxu(root->right,i,ans,k);
    }
    int kthSmallest(TreeNode* root, int k) {
        int i=1,ans;
        zhongxu(root,i,ans,k);
        return ans;
    }
};

非递归

/**
 * 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) {
        vector<int>v;
        stack<TreeNode*>s;
        TreeNode* p;
        
        s.push(root);
        while(!s.empty()){
            while(p=s.top())  // 向左走到尽头
                 s.push(p->left);
            s.pop();// 空指针出栈
            if(!s.empty()){
                p=s.top();//取栈顶元素
                s.pop();//栈顶元素出栈
                v.push_back(p->val);//加到向量vector中
                s.push(p->right);
            }
        }
        return v[k-1];
    }
};

leetcode-230-Kth Smallest Element in a BST

标签：leetcode

原文地址：http://blog.csdn.net/u014705854/article/details/46815165