187. Repeated DNA Sequences

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本文介绍了一种高效的方法来查找DNA序列中重复出现的10字母长的子序列。通过将DNA字母映射到数值并利用哈希技巧，可以快速识别出重复的序列。

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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"].

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;
    }
};