文章作者：Tyan
博客：noahsnail.com  |  CSDN  |  简书

1. Description

Insert into a Binary Search Tree

2. Solution

• Iterative

/**
 * 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:
    TreeNode* insertIntoBST(TreeNode* root, int val) {
        TreeNode* insert = new TreeNode(val);
        if(!root) {
            root = insert;
            return root;
        }
        TreeNode* current = root;
        TreeNode* pre = NULL;
        while(current) {
            pre = current;
            if(current->val > val) {
                current = current->left;
            }
            else {
                current = current->right;
            }
        }
        if(pre->val > val) {
            pre->left = insert;
        }
        else {
            pre->right = insert;
        }
        return root;
    }
};

• Recursive

/**
 * 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:
    TreeNode* insertIntoBST(TreeNode* root, int val) {
        TreeNode* insert = new TreeNode(val);
        if(!root) {
            root = insert;
            return root;
        }
        insertNodeIntoBST(root, val);
        return root;
    }
    
    void insertNodeIntoBST(TreeNode* root, int val) {
        if(root->val > val) {
            if(!root->left) {
                root->left = new TreeNode(val);
            }
            else {
                insertNodeIntoBST(root->left, val);
            }
        }
        else {
            if(!root->right) {
                root->right = new TreeNode(val);
            }
            else {
                insertNodeIntoBST(root->right, val);
            }
        }
    }
};

Reference

1. https://leetcode.com/problems/insert-into-a-binary-search-tree/description/