LeetCode 230 Kth Smallest Element in a BST 二叉搜索树中的第K个元素

1、非递归解法

/**
 * 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) {
        if(root==nullptr)
            return -1;
        stack<TreeNode*> stk;
        while(root||!stk.empty())
        {
            if(root)
            {
                stk.push(root);
                root=root->left;
            }
            else
            {
                root=stk.top();
                stk.pop();
                if(k==1)
                    return root->val;
                --k;
                root=root->right;
            }
        }
        return -1;
    }
};

2、递归解法

/**
 * 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) {
        return helper(root, k);
    }
    int helper(TreeNode* root, int &k) {
        if (!root)
            return -1;
        int val = helper(root->left, k);
        if (k == 0)
            return val;
        if (--k == 0)
            return root->val;
        return helper(root->right, k);
    }
};