【C语言刷力扣】LCP 67.装饰树

题目：

解题思路：

    递归实现后序遍历

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     struct TreeNode *left;
 *     struct TreeNode *right;
 * };
 */

struct TreeNode* expandBinaryTree(struct TreeNode* root){
    if(root == NULL) return root;

    expandBinaryTree(root -> left);
    expandBinaryTree(root -> right);

    if(root -> left != NULL) {
        struct TreeNode* newL = (struct TreeNode*) malloc (sizeof(struct TreeNode));
        newL -> val = -1;
        newL -> left = root -> left;
        root -> left = newL;
        newL -> right = NULL;
    }
    if(root -> right != NULL) {
        struct TreeNode* newR = (struct TreeNode*) malloc (sizeof(struct TreeNode)); 
        newR -> val = -1;
        newR -> right = root ->right;
        root -> right = newR;
        newR -> left = NULL;
    }
    return root;
}