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;
	}
};