LeetCode | 二叉树的前中后序遍历

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 TreeSize(struct TreeNode* root)
{
    return root == NULL ? 0 : TreeSize(root->left) + TreeSize(root->right) + 1;
}
int* preorderTraversal(struct TreeNode* root, int* returnSize) {
    //计算树有多少个节点
    int n = TreeSize(root);
    *returnSize = n;
    //开辟n个大小
    int* a = malloc(sizeof(int) * n);
    int i = 0;
    //前序遍历
    preorder(root,a,&i);
    return a;
}