LeetCode - Repeated DNA Sequences

LeetCode -- Repeated DNA Sequences
题目描述：


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"].




就是在一个字符串s中长度为10的字串中找到出现次数大于1次的子串。


思路：
使用哈希统计s[i...i+10]出现的次数，i∈[0,n-9)。




实现代码：

public class Solution {
    public IList<string> FindRepeatedDnaSequences(string s) 
    {
        if(s.Length < 11){
    		return new List<string>();
    	}
    	var hash = new Dictionary<string ,int>();
    	for(var i = 0;i < s.Length - 9; i++){
    		var t = s.Substring(i,10);
    		if(!hash.ContainsKey(t)){
    			hash.Add(t,1);
    		}
    		else{
    			hash[t]++;
    		}
    	}
    	
    	var ret = new List<string>();
    	foreach(var k in hash.Keys){
    		if(hash[k] > 1){
    			ret.Add(k);
    		}
    	}
    	
    	return ret;
    }
}