Day_03：（LeetCode）二叉树的前序遍历

链接：

https://leetcode-cn.com/problems/binary-tree-preorder-traversal/description/

问题描述：

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

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

int* preorderTraversal(struct TreeNode* root, int* returnSize)
{
    *returnSize = GetTreeSize(root);
    int* array = (int*)malloc(*returnSize*sizeof(int));
    int Index;
    _preorderTraversal(root, array, &Index);
    
    return array;
}