Leetcode-14. Longest Common Prefix

Problem Description:

Write a function to find the longest common prefix string amongst an array of strings.

Analysis:

It’s an easy problem. First pieces of code is regular solution.

I try to use Trie (prefix tree) to solve the problem. Notice that, Trie is very helpful in prefix matching problem.

string longestCommonPrefix(vector<string>& strs) {
if (strs.empty()) return "";
string lcp = "";
int m = strs.size(), n = strs[0].size();
for (int i = 0; i < n; ++i){
for (int j = 1; j < m; ++j)
if (strs[j][i] != strs[0][i] || strs[j].size() == i)
return lcp;
lcp += strs[0][i];
}
return lcp;
}

Using Trie data structure:

class TrieNode
{
public:
TrieNode* next[52];//upper and lower case
bool end_Word;     // end of a word
int sons;         // numbers of children
TrieNode() : end_Word(false), sons(0) //
{
for (int i = 0 ; i < 52; ++i)
next[i] = NULL;
}
};
class Trie
{
public:
TrieNode * root;
Trie ()
{
root = new TrieNode();
}
void insert(string word) {
TrieNode * p = root;
for (char c : word)
{
if (c >= 'a')
{
if (p -> next[c - 'a'] == NULL)
{
p -> next[c - 'a'] = new TrieNode();
p -> sons ++; //
}
p = p -> next[c - 'a'];
}
else
{
if (p -> next[c - 'A' + 26] == NULL)
{
p -> next[c - 'A' + 26] = new TrieNode();
p -> sons ++;
}

p = p -> next[c - 'A' + 26];
}
}
p -> end_Word = true;
}
};

class Solution {
public:
string longestCommonPrefix(vector<string>& strs)
{
Trie T;
for(auto str : strs)
T.insert(str);
return findPrefix(T);
}
private :
string findPrefix(Trie& T)
{
TrieNode* p = T.root;
string res = "";
int i = 0;
while (p -> sons == 1 && !p -> end_Word)
{
for (i = 0; i < 52; ++i)
{
if (p -> next[i])
{
if (i < 26)
res += char('a' + i);
else
res += char('A' + i - 26);
break;
}
}
if (i == 52) break;
p = p -> next[i];
}
return res;
}
};

Python code:

class Solution(object):
def longestCommonPrefix(self, strs):
"""
:type strs: List[str]
:rtype: str
"""
sz, ret = zip(*strs), ""
# zip('ABCD', 'xy') --> Ax By
for c in sz:
if len(set(c)) > 1: break
ret += c[0]
return ret