二叉树的中序遍历

给定一个二叉树，返回它的中序 遍历。

示例:

输入: [null,1,3,2,null]
   1
    \
     2
    /
   3

输出: [1,3,2]

https://leetcode-cn.com/problems/binary-tree-inorder-traversal/description/
递归访问

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     struct TreeNode *left;
 *     struct TreeNode *right;
 * };
 */
/**
 * Return an array of size *returnSize.
 * Note: The returned array must be malloced, assume caller calls free().
 */

int TreeNodeSize(struct TreeNode* root)
{
    if(NULL == root)
        return 0;
    return 1 + TreeNodeSize(root->left) + TreeNodeSize(root->right);
}

void _inorderTraversal(struct TreeNode *root,int *array,int *pIndex)
{
    if(NULL == root)
        return ;
    _inorderTraversal(root->left,array,pIndex);
    array[*pIndex]=root->val;
    (*pIndex)++;
    _inorderTraversal(root->right,array,pIndex);
}

int* inorderTraversal(struct TreeNode* root, int* returnSize) {
    *returnSize = TreeNodeSize(root);
    int *array = (int *)malloc(*returnSize * sizeof(*returnSize));
    int index = 0;
    _inorderTraversal(root,array,&index);
    return array;
}