POJ 1056 IMMEDIATE DECODABILITY

题目大意：

给定多组字符串，对每组字符串，如果存在至少一个字符串是其他某些字符的前缀，那么该组字符串是不能immediately decodable，否则是能够immediately decodable。

此题如果利用传统的搜索算法，在时间复杂度上不能很好地满足题目要求，因此考虑采用字典树实现。具体实现程序如下：

#include <iostream>
#include <string>
using namespace std;

struct TrieNode
{
	int count;
	struct TrieNode *branch[2];
	TrieNode()
	{
		count = 0;
		for(int i = 0; i < 2; i++)
		{
			branch[i] = NULL;
		}
	}
};

//将字符串word插入根为root的字典树
void insert(TrieNode *&root, string word)
{
	if(root == NULL)
	{
		root = new TrieNode();
	}
	TrieNode *p = root;
	for(int i = 0; i < word.size(); i++)
	{
		if(!p->branch[word[i] - '0'])
		{
			p->branch[word[i] - '0'] = new TrieNode();
		}
		p = p->branch[word[i] - '0'];
		p->count++;
	}
}

//判断字符串word是否为其他字符串的前缀
//如果是其他字符串的前缀返回false，否则返回true
bool isPrefix(TrieNode *root, string word)
{
	if(root == NULL)
	{
		return false;
	}
	string str = "";
    TrieNode *p = root;
	for(int i = 0; i < word.size(); i++)
	{
        p = p->branch[word[i] - '0'];
		if(p->count == 1)
			return true;
    }

	return false;
}

int main()
{
	
	int k = 0, i, num = 1;
	string str, words[8];
	TrieNode *root = NULL;
	while(cin >> str)
	{
		if(str == "9")
		{
			bool flag = true;
			for(i = 0; i < k; i++)
			{
				if(!isPrefix(root, words[i]))
				{
					flag = false;
					break;
				}
			}
			if(flag)
			{
				cout << "Set " << num << " is immediately decodable" << endl;
			}
			else
			{
				cout << "Set " << num << " is not immediately decodable" << endl;
			}
			num++;
			k = 0;
			root = NULL;
		}
		else
		{
			words[k] = str;
			insert(root, words[k]);
			k++;
		}
	}

	return 0;
}