为什么会出问题 1个MLE,九个WE 请大佬指正！！！

#include <iostream>
using namespace std;

struct Tree{
    int left,right;
}tree[100001];

void preorder(int n){
	cout << n;
	if(tree[n].left == 0)
		preorder(tree[n].left);
	if(tree[n].right == 0)
		preorder(tree[n].right);
}

void inorder(int n){
	if(tree[n].left == 0)
		inorder(tree[n].left);
	cout << n;
	if(tree[n].right == 0)
		inorder(tree[n].right);
}

void postorder(int n){
	if(tree[n].left == 0)
		postorder(tree[n].left);
	if(tree[n].right == 0)
		postorder(tree[n].right);
	cout << n;
}

int main(){
    int num;
    cin >> num;
    for(int i=1; i<=num; i++){
        cin >> tree[i].left >> tree[i].right;
    }
    preorder(1);
    inorder(1);
    postorder(1);
    return 0;
}