230题

方法

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    int i=0;
    int temp;
    int kthSmallest(TreeNode* root, int k) {
        if(root==nullptr){
            return 0;
        }
        kthSmallest(root->left,k);
        i++;
        if(i==k){
            temp=root->val;
        }
        kthSmallest(root->right,k);
        return temp;
    }
};

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    int kthSmallest(TreeNode* head, int k) {
        stack<TreeNode*>s;
        while(!s.empty()||head!=nullptr){
            if(head){
                s.emplace(head);
                head=head->left;
            }else{
                head=s.top();
                s.pop();
                k--;
                if(k==0){
                    return head->val;
                    break;
                }
                head=head->right;
            }
        }
        return 0;
    }
};