187. Repeated DNA Sequences (String; Bit)

 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"].

 思路I：遍历string，每次截取10个字符，判断出现次数。

Result: Time Limit Exceeded

思路II：字符数较少=>用数字表示字符=>用bitmap来表示字符串，好处：节省空间

比如本题只可能出现4种字符=>可表示为0，1，2，3，即可以用2bits来表示=>字符原本一个字符占1 byte = 8 bits，现在只要2 bits

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) {
        int sLen = s.length();
        unsigned int val=0;
        char mp[1024*1024]={0};
        vector<string> ret;
        string str;
        
        if(sLen < 10) return ret;
        
        for(int i = 0; i < 9; i++){
            val <<=2;
            val |= getVal(s[i]);
        }
        
        for(int i = 9; i < sLen; i++){
            val <<= 2;
            val |= getVal(s[i]);
            val &= 0xFFFFF;
            if(++mp[val] == 2){
                str = s.substr(i-9,10);
                ret.push_back(str);
            }
        }

        return ret;
    }
};