每天一道LeetCode-----寻找二叉搜索树中第k小的元素

Kth Smallest Element in a BST

/**
* 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) {
int res = 0;
inOrder(root, k, res);
return res;
}
private:
void inOrder(TreeNode* root, int& k, int& res)
{
if(!root || !k) return;
inOrder(root->left, k, res);
//找到k个数即可
if(--k == 0)
res = root->val;
inOrder(root->right, k, res);
}
};

/**
* 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) {
stack<TreeNode*> s;
pushAllLeft(root, s);
TreeNode* top = nullptr;
while(k--)
{
top = s.top();
s.pop();
//每弹出一个节点就将该节点的右子树的左边一列入栈
pushAllLeft(top->right, s);
}
return top->val;
}
private:
void pushAllLeft(TreeNode*root, stack<TreeNode*>& s)
{
while(root)
{
s.push(root);
root = root->left;
}
}
};