230-二叉搜尋樹中第K小的元素

遞歸版：

/**
 * 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* root, int k) {
        vector<int>arr;
        dfs(root, k, arr);
        return arr[k - 1];
    }
    void dfs(TreeNode *root, int k, vector<int>&arr) {
        if (root == NULL) {
            return;
        }
        dfs(root->left, k, arr);
        arr.push_back(root->val);
        dfs(root->right, k, arr);
    }
};
           

非遞歸版：

/**
 * 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* root, int k) {
        stack<TreeNode *>stk;
        int num = 0;
        TreeNode *node = root;
        while (node || !stk.empty()) {
            while (node != NULL) {
                stk.push(node);
                node = node->left;
            }
            if (!stk.empty()) {
                node = stk.top();
                k--;
                if (k == 0) {
                    return node->val;
                }
                stk.pop();
                node = node->right;
            }
        }
        return 0;
    }
};