Kth Smallest Element in a BST

給頂一個二叉搜尋樹的根節點，找到這棵數第k小的值

兩種方法

• 遞迴法的中序遍歷
• 迭代法的中序遍歷

遞迴法，常規的中序遍歷

程式碼如下

/**
 * 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個數即可
 

        if(--k == 0)
            res = root->val;
        inOrder(root->right, k, res);
    }
};

迭代法，用棧實現

/**
 * 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();
            //每彈出一個節點就將該節點的右子樹的左邊一列入棧
            pushAllLeft(top->right, s);
        }
        return top->val;
    }
private:
    void pushAllLeft(TreeNode*root, stack<TreeNode*>& s)
    {
        while(root)
        {
            s.push(root);
            root = root->left;
        }
    }
};

本題主要考察中序遍歷，除了遞迴法最好可以熟悉迭代法的求解