[LeetCode] 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"].

用Map的话超内存了，改用bitsmap，因为只有4个字母，所以只要用两位就可以做为一个字母的编码，10个字母就是20位，所以创建一个2^20大小的数组就可以解决问题了。

class Solution {
public:
    int getVal(char ch) {
        if (ch == 'A') return 0;
        if (ch == 'C') return 1;
        if (ch == 'G') return 2;
        if (ch == 'T') return 3;
    }
    
    vector<string> findRepeatedDnaSequences(string s) {
        set<string> st;
        vector<string> res;
        string str;
        if (s.length() < 10 || s == "") return res;
        int mp[1024*1024] = {0};
        unsigned int val = 0;
        for (int i = 0; i < 9; ++i) {
            val <<= 2;
            val |= getVal(s[i]);
        }
        for (int i = 9; i < s.length(); ++i) {
            val <<= 14;
            val >>= 12;
            val |= getVal(s[i]);
            ++mp[val];
            if (mp[val] > 1) {
                str = s.substr(i-9, 10);
                st.insert(str);
            }
        }
        for (set<string>::iterator i = st.begin(); i != st.end(); ++i) {
            res.push_back(*i);
        }
        return res;
    }
};