LeetCode刷題筆記 230. 二叉搜尋樹中第K小的元素

• 遞歸
• 疊代

二叉搜尋樹中第K小的元素

遞歸

class Solution {
private:
    int n=0,res;
public:
    void dfs(TreeNode* root,int k){
        if(!root) return;
        dfs(root->left,k);
        n++;
        if(n==k) res=root->val;
        dfs(root->right,k);
    }
    int kthSmallest(TreeNode* root, int k) {
        dfs(root,k);
        return res;
    }
};
           

疊代

class Solution {
public:
    int kthSmallest(TreeNode* root, int k) {
        stack<TreeNode*> s;
        int res,n=0;
        TreeNode *cur=root;
        while(!s.empty()||cur){
            while(cur){
                s.push(cur);
                cur=cur->left;
            }
            cur=s.top();
            s.pop();
            n++;
            if(n==k) return cur->val;
            cur=cur->right;
        }
        return 0;
    }
};