187. Repeated DNA Sequences

All DNA is composed of a series of nucleotides abbreviated as A, C, G, and T, for example: “ACGAATTCCG”. When studying DNA, it is sometimes useful to identify repeated sequences within the DNA.

Write a function to find all the 10-letter-long sequences (substrings) that occur more than once in a DNA molecule.

For example,

Given s = "AAAAACCCCCAAAAACCCCCCAAAAAGGGTTT",

Return:
["AAAAACCCCC", "CCCCCAAAAA"].

先献出自己的菜鸡想法，利用一个map存储每10个子元素组成的字串，并++，最后在map中找出出现次数大于1的字串；

vector<string> findRepeatedDnaSequences(string s) {
    if (s.size() < 10)return vector<string>{};
    unordered_map<string, int> temp;
    vector<string> res;
    int len = s.size();
    for (int i = 0; i <= len - 10; i++){
        temp[s.substr(i, 10)]++;
    }
    for (auto x : temp){
        if (x.second > 1)res.push_back(x.first);
    }
    return res;
}

写完之后自己都感觉的弱鸡的不行，这样带来的空间复杂度可能会很大，但还好时间复杂度依旧是O（N），不过还是不理想；

思路2：LeetCode上参考的代码，主要运用了位操作，其实也没有什么太过于神奇的地方，但是空间复杂度明显下降。

int char2val(char c){
    switch (c){
    case 'A':return 0;
    case 'C':return 1;
    case 'G':return 2;
    case 'T':return 3;
    default:return 0;
    }
}

vector<string> findRepeatedDnaSequences(string s) {
    if (s.size() < 10)return vector<string>{};
    vector<string> res;
    bitset<1 << 20> s1, s2;
    int mask = (1 << 20) - 1;
    int val = 0;
    for (int i = 0; i < 10; i++){
        val = (val << 2) | char2val(s[i]);
    }
    s1.set(val);
    for (int i = 10; i < s.size(); i++){
        val = ((val << 2) & mask) | char2val(s[i]);
        if (s2[val])continue;
        if (s1[val]){
            res.push_back(s.substr(i - 10 + 1, 10));
            s2.set(val);
        }
        else{
            s1.set(val);
        }
    }
    return res;
}