[LeetCode]Repeated DNA Sequences

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"].

首先想到的是用Hash Table判断只要O(n)的时间复杂度。但是内存超过限制，原因是String作为key占用了很大的内存。
解决方法就是二进制编码，因为只有4个字母，可以A=00，C=01，G=10，T=11。十位字母也就20位，而int有4个字节，32位，完全够用。
这样，我们就可以使用int表示一个string，节省了大量的内存。

class Solution {
public:
    vector<string> findRepeatedDnaSequences(string s) {
        vector<string> result;
        if(s.length()<=10) return result;
        unordered_map<int,int> showed;
        for(int i=0;i<=s.length()-10;i++)
        {
            string temp_str = s.substr(i,10);
            int temp = 0;
            for(int i=0;i<10;i++)
            {
                if(temp_str[i]=='A') temp = temp*4+0;
                else if(temp_str[i]=='C') temp = temp*4+1;
                else if(temp_str[i]=='G') temp = temp*4+2;
                else temp = temp*4+3;
            }
            if(showed.find(temp)!=showed.end())
            {
                if(showed[temp]==1)
                {
                    result.push_back(temp_str);
                    showed[temp]++;
                }
                else
                {
                    showed[temp]++;
                }
            }
            else
            {
                showed[temp]=1;
            }
        }
        return result;
    }
};