2023.7.12

         比较简单，不多说直接看代码：

迭代法：

class Solution {
public:
    TreeNode* insertIntoBST(TreeNode* root, int val) {
        //树为空的情况
        if(root==nullptr)
        {
            TreeNode* node = new TreeNode(val);
            return node;
        }
        //需要一个pre节点指向父节点
        TreeNode* cur = root;
        TreeNode* pre = root;
        //遍历，若cur值比val小，则cur往右走；反之
        while(cur)
        {
            pre = cur;
            if(cur->val < val) cur = cur->right;
            else cur = cur->left;
        }
        //此时cur为空了，pre为cur父节点，最后判断一下大小执行插入操作
        TreeNode* node = new TreeNode(val);
        if(pre->val > val) pre->left = node;
        else pre->right = node;
        return root;                     
    }
};

递归法：

class Solution {
public:
    TreeNode* insertIntoBST(TreeNode* root, int val) {
        if(root==nullptr)
        {
            TreeNode* node = new TreeNode(val);
            return node;
        }
        if(root->val < val) root->right = insertIntoBST(root->right,val);
        if(root->val > val) root->left = insertIntoBST(root->left,val);
        
        return root;
    }
};