leetcode144.二叉树的前序遍历

/**
 * 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;
 }
void preorder(struct TreeNode* root,int* a,int* pi)
{
    if(root==NULL)
        return;
    a[(*pi)++]=root->val;
    preorder(root->left, a, pi);
    preorder(root->right, a, pi);
}
int* preorderTraversal(struct TreeNode* root, int* returnSize){
    *returnSize=TreeSize(root);
    int* a=(int*)malloc(*returnSize * sizeof(int));
    int i = 0;
    preorder(root, a, &i);
    return a;
}