leetcode 720. Longest Word in Dictionary

720. Longest Word in Dictionary

Given a list of strings words representing an English Dictionary, find the longest word in words that can be built one character at a time by other words in words. If there is more than one possible answer, return the longest word with the smallest lexicographical order.

If there is no answer, return the empty string.

Example 1:

Input: 
words = ["w","wo","wor","worl", "world"]
Output: "world"
Explanation: 
The word "world" can be built one character at a time by "w", "wo", "wor", and "worl".

Example 2:

Input: 
words = ["a", "banana", "app", "appl", "ap", "apply", "apple"]
Output: "apple"
Explanation: 
Both "apply" and "apple" can be built from other words in the dictionary. However, "apple" is lexicographically smaller than "apply".

Note:

• All the strings in the input will only contain lowercase letters.
• The length of words will be in the range [1, 1000].
• The length of words[i] will be in the range [1, 30].

  题目的意思是看word的所有前缀在不在 集合里面。

class TrieNode{  
public:  
    char var;  
    bool isWord;  
    TrieNode* children[26];  
      
    TrieNode()     
    {      
        var = 0;      
        isWord = false;      
        memset(children, 0, sizeof(TreeNode*)*26);      
      
    }      
    TrieNode(char c)    
    {      
        var = c;      
        isWord = false;      
        memset(children, 0, sizeof(TreeNode*)*26);      
    }      
};  

static bool compare(string a, string b)
{
    if (a.size() == b.size())
    {
        int k = 0;
        while (k < a.size() && k < b.size() && a[k] == b[k])
            k++;
        return a[k] < b[k];
    }
    return a.size() > b.size();
}

class Solution {
public:
    string longestWord(vector<string>& words) 
    {
        root = new TrieNode();  
        //构造字典树  
        for (auto s : words)  
        {  
            TrieNode* p = root;  
            for (int k = 0; k < s.size(); k++)  
            {  
                if (p->children[s[k] - 'a'] == 0)  
                {  
                    TrieNode* pnew = new TrieNode(s[k]);  
                    p->children[s[k] - 'a'] = pnew;  
                }  
                p = p->children[s[k] - 'a'];  
            }  
            p->isWord = true;  
        }  
        vector<string> ret;
        int maxretsize = 0; //记录已经得到的ret中最长的
        for (auto s : words)
        {
            if (s.size() < maxretsize) continue;
            TrieNode* p = root;
            for (int k = 0; k < s.size(); k++)
            {
                p = p->children[s[k] - 'a'];
                if (!p->isWord)
                    break;
                if (k + 1 == s.size()) //最后一位了
                {    
                    ret.push_back(s);       
                    maxretsize = s.size();
                }
            }
        }
        sort(ret.begin(), ret.end(), compare);
        return ret.empty() ? "" : ret[0];
    }
private:
    TrieNode* root;
};