[Leetcode] 187. Repeated DNA Sequences 解题报告

题目：

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：

不知道这道题目为什么难度被定义为了medium。其实好像也没有用到medium级别的算法，我们只是定义了一张哈希表，然后扫描数组，在哈希表里面查找即可。这样的算法也通过了。

代码：

class Solution {
public:
    vector findRepeatedDnaSequences(string s) {
        vector ret;
        if(s.size() < 10) {
            return ret;
        }
        unordered_map hash;  
        for(int i = 0; i < s.size() - 9; ++i) {  
            string str = s.substr(i, 10);  
            if(hash.count(str) && hash[str] < 2) {
                ret.push_back(str);  
            }
            hash[str]++;  
        }  
        return ret;  
    }
};