# O(N) algorithm in C++

• The logic is, traverse the string, once you meet a new char, find it in hash table
It means it's a repeated one if you can find one, recalculate the length of new substring. It may be the current length+1 or have to be recalculate. Take the minimum of two.

If you cannot find it in hash table, then it simply means that it's not a repeated one. So just add one to current length

``````int lengthOfLongestSubstring(string s) {

size_t max_count = 0, new_count = 0;
unordered_map<char,size_t> myMap;

for (size_t i = 0 ; i < s.size(); i++) {
auto it = myMap.find(s[i]);
if ( it == myMap.end()) {// cannot find, put in hashtable and increase substr length
new_count ++;
if( new_count > max_count)
max_count = new_count;
}
else { // can find, find the start, recalculate the truncated substr length
new_count = min( i-myMap[s[i]], new_count +1);
max_count = max(max_count, new_count);
}
myMap[s[i]] = i;

}
return max_count;
}``````

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