leetcode：前序遍历

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     struct TreeNode *left;
 *     struct TreeNode *right;
 * };
 */
/**
 * Note: The returned array must be malloced, assume caller calls free().
 */
int TreeSize(struct TreeNode* root)
{
    return root==NULL?0:TreeSize(root->left)+TreeSize(root->right)+1;
}
//int i=0;
void preorder(struct TreeNode*root,int *a,int *i)
{
    if(root==NULL)
        return;
    a[(*i)++]=root->val;
    preorder(root->left,a,i);
    preorder(root->right,a,i);
}
int* preorderTraversal(struct TreeNode* root, int* returnSize) {
    int n=TreeSize(root);
    int *a=(int*)malloc(sizeof(int)*n);
    *returnSize=n;
    int i=0;
    preorder(root,a,&i);
    return a;
}