LeetCode之Repeated DNA Sequences

/*本题最直观的想法是用Hash Table存储起连续的10个字符串，但是这样会超出内存限制。
所以用一个数字代替A/C/GT，减少内存消耗。*/
class Solution {
public:
vector<string> findRepeatedDnaSequences(string s) {
vector<int> letters(256, 0);
letters['A'] = 0;
letters['C'] = 1;
letters['G'] = 2;
letters['T'] = 3;
vector<string> res;
unordered_map<int, int> hash;
int key = 0, er = 0xFFFFF;
for(int i = 0; i < s.size(); ++i){
key = (key << 2) + letters[s[i]];
if(i >= 9){
key &= er;
if(hash[key] == 1) res.push_back(s.substr(i-9, 10));
hash[key] += 1;
}
}
return res;
}
};