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"].

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找出给定字符串中长度为10的有重复的子字符串。如果单纯字符串的比较的话肯定是会超时的，要想办法对字符串进行编码。刚好字符串只有ACGT四种字符，所以定为0、1、2、3，每十个字符求一个值，判断该值是否出现过一次，就可以知道是不是重复的了。

代码：

class Solution {public:    vector<string> findRepeatedDnaSequences(string s) {    vector<string> res;    if(s.empty() || s.size() < 11) return res;        char char_map[127];        char_map['A'] = 0;        char_map['C'] = 1;        char_map['G'] = 2;        char_map['T'] = 3;        map<int, int> nums;        for(int i = 0; i <= s.size()-10; ++i)        {        int num = 0;        for(int j = i; j < i+10; ++j)        {        num = num * 10 + char_map[s[j]];}if(nums[num]++ == 1){res.push_back(s.substr(i, 10));}}return res;    }};