pat 1110. Complete Binary Tree (25)

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本文介绍了一种检查二叉树是否按层序遍历的方法，并提供了两种实现代码。通过标记节点是否有子节点来判断遍历过程是否有效。

https://www.patest.cn/contests/pat-a-practise/1110

层序遍历，如果没有child时打标记，如果再出现inValid

#include <iostream>
#include <string>
#include <cstdio>
#include <cstring>
#include <cstdlib>
#include <queue>
#define MAXN 21

using namespace std;
typedef struct node {
	int v, l, r;
	node() {
		v = -1,l = -1, r = -1;
	}
}Node;
int father[MAXN];
Node nodes[MAXN];
int n;

int main()
{
	string sl, sr;
	int l, r;
	cin >> n;
	memset(father, -1, sizeof(father));
	for (int i = 0; i < n; i++)
	{
		cin >> sl >> sr;
		nodes[i].v = i;
		if (sl != "-") {
			l = stoi(sl.c_str());
			father[l]  = i;
			nodes[i].l = l;
		}
		if (sr != "-") {
			r = stoi(sr.c_str());
			father[r] = i;
			nodes[i].r = r;
		}
	}
	
	int root;
	for (int i = 0; i < n; i++)
	{
		if (father[i] == -1) root = i;
	}
	bool  nochild = false,inValid = false;
	queue<Node> q;
	q.push(nodes[root]);
	int lastnode = root;
	Node node;
	while (!q.empty()) {
		node = q.front();
		q.pop();

		if (node.l != -1) {
			if (nochild) {
				inValid = true; break;
			}
			q.push(nodes[node.l]);
			lastnode = node.l;
		}
		else {
			nochild = true;
		}

		if (node.r != -1) {
			if (nochild) {
				inValid = true; break;
			}
			q.push(nodes[node.r]);
			lastnode = node.r;
		}
		else {
			nochild = true;
		}
	}
	if (!inValid) printf("YES %d\n",lastnode);
	else printf("NO %d\n", root);
	return 0;
}

另附一份“聪明”的代码：

http://www.liuchuo.net/archives/2158

#include <cstdio>
#include <queue>
#include <vector>
#include <string.h>
using namespace std;
struct TREE {
    int left;
    int right;
};
int main() {
    int n, root = 0;
    scanf("%d", &n);
    vector<TREE> tree(n);
    vector<int> book(n);
    for(int i = 0; i < n; i++) {
        char l[3], r[3];
        scanf("%s %s", l, r);
        if(l[0] == '-') {
            tree[i].left = -1;
        } else {
            int temp = l[0] - '0';
            if(strlen(l) == 2) {
                temp = temp * 10 + (l[1] - '0');
            }
            tree[i].left = temp;
        }
        if(r[0] == '-') {
            tree[i].right = -1;
        } else {
            int temp = r[0] - '0';
            if(strlen(r) == 2) {
                temp = temp * 10 + (r[1] - '0');
            }
            tree[i].right = temp;
        }
        if(tree[i].left != -1)
            book[tree[i].left] = 1;
        if(tree[i].right != -1)
            book[tree[i].right] = 1;
    }
    for(int i = 0; i < n; i++) {
        if(book[i] == 0) {
            root = i;
            break;
        }
    }
    queue<int> q;
    q.push(root);
    int cnt = 0;
    int lastnode = 0;
    while(!q.empty()) {
        int node = q.front();
        q.pop();
        if(node != -1) {
            lastnode = node;
            cnt++;
        }else {
            if(cnt != n)
                printf("NO %d", root);
            else
                printf("YES %d", lastnode);
            return 0;
        }
        q.push(tree[node].left);
        q.push(tree[node].right);
    }
    return 0;
}