int* preorderTraversal( struct TreeNode*root, int* returnSize) {
}

解答代码：

int TreeSize(struct TreeNode*root)
 {
     return root==NULL?0
     :TreeSize(root->left)+TreeSize(root->right)+1;
 }
 void Prevorder(struct TreeNode*root,int*a,int*pi)
 {
    if(root==NULL)
    return;
    a[(*pi)++]=root->val;
    Prevorder(root->left,a,pi);
    Prevorder(root->right,a,pi);
 }
int* preorderTraversal( struct TreeNode*root, int* returnSize) {
    int n=TreeSize(root);
    int *a=(int*)malloc(sizeof(int)*n);
    *returnSize=n;
    int i=0;
    Prevorder(root,a,&i);
    return a;
}

ps:
题目里给的形参：int*returnSize 是个输出型变量 ，之所以给指针，是为了便于修改returnSize所指向的数值,这个数值就是数组中元素的个数