701. Insert into a Binary Search Tree

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解法一：递归

/**
 * 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) {
        if(root == NULL){
            return new TreeNode(val);
        }
        if(val < root->val){
            root->left = insertIntoBST(root->left, val);
        }
        else{
            root->right = insertIntoBST(root->right, val);
        }
        return root;
    }
};

解法二：迭代
!!!也很好理解，要注意，当找到合适的位置时，要及时返回。

/**
 * 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) {
        if(root == NULL) return new TreeNode(val);
        TreeNode* tmp = root;
        while(tmp){
            if(tmp->val > val){
                if(tmp->left == NULL){
                    tmp->left = new TreeNode(val);
                    return root;
                }
                tmp = tmp->left;
            }
            else{
                if(tmp->right == NULL){
                    tmp->right = new TreeNode(val);
                    return  root;
                }
                tmp = tmp->right;
            }
        }
        return root;
    }
};