LeetCode 230 Kth Smallest Element in a BST

LeetCode 230 Kth Smallest Element in a BST

思路

中序遍历

代码

迭代

class Solution {
public:
    int kthSmallest(TreeNode* root, int k) {
        stack<TreeNode*> stk;
        TreeNode *cur = root;
        
        
        while (cur || !stk.empty()) {
            while (cur) {
                stk.push(cur);
                cur = cur->left;
            }
            
            cur = stk.top();
            stk.pop();
            if (!--k) 
                return cur->val;
            cur = cur->right;
            
        }
        
        return cur->val;
    }
};

递归

class Solution {
public:
    int kthSmallest(TreeNode* root, int k) {
        vector<int> ans;
        inorder(root, ans);
        return ans[k-1];
    }
    
    void inorder(TreeNode *root, vector<int> &ans) {
        if (!root) return ;
        inorder(root->left, ans);
        ans.push_back(root->val);
        inorder(root->right, ans);
    }
};