leetcode701. Insert into a Binary Search Tree

https://www.cnblogs.com/grandyang/p/9914546.html 

类似于二分查找的方法，用迭代的方法去做

注意：无论是进入左子树还是右子树，左右子树都变成了新的数，所以需要重新根据root->left = ....来重新生成

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