1138 Postorder Traversal（25 分）

看到节点这么多，以为递归进去会超时，但是并没有，四次方递归不会超时，心里记一下
模板题目，后序和中序重建二叉树，之后后序遍历

#include
#include
using namespace std;
const int maxn = 5e4 + 10;
int pre[maxn], in[maxn];
struct node {
    int data;
    node *lchild, *rchild;
};
vectorans;
node* create(int preL, int preR, int inL, int inR)
{
    if (preL > preR)return NULL;
    node* root = new node;
    root->data = pre[preL];
    int k;
    for (k = inL; k <= inR; k++)
    {
        if (in[k] == root->data)break;
    }
    int numleft = k - inL;
    root->lchild = create(preL + 1, preL + numleft, inL, k - 1);
    root->rchild = create(preL + numleft + 1, preR, k + 1, inR);
    return root;
}
void postorder(node* root)
{
    if (root == NULL)return;
    postorder(root->lchild);
    postorder(root->rchild);
    ans.push_back(root->data);
}
int main()
{
    int n;
    scanf("%d", &n);
    for (int i = 0; i < n; i++)scanf("%d", &pre[i]);
    for (int i = 0; i < n; i++)scanf("%d", &in[i]);
    node*root = create(0, n - 1, 0, n - 1);
    postorder(root);
    printf("%d", ans[0]);
    return 0;
}