[leetcode]187. Repeated DNA Sequences重复DNA序列

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.

Example:

Input: s = "AAAAACCCCCAAAAACCCCCCAAAAAGGGTTT"

Output: ["AAAAACCCCC", "CCCCCAAAAA"]

题意：

DNA序列里有ATCG四种，求所有长度为10，出现次数超过一次的序列。

Solution1: HashMap

1.  scan the given string,  put each 10-letter-long substring and its corresponding frequency into a map

2. looping each entrySet in the map, find if entry.getValue() > 1 

code

/*
Time Complexity: O(n)
Space Complexity: O(n)
*/
class Solution {
    public List<String> findRepeatedDnaSequences(String s) {
        List<String> result = new ArrayList<>();
        // corner case
        if (s.length() < 10) return result;
        
        Map<String, Integer> map = new HashMap<>();
        for (int i = 0; i < s.length() - 9; ++i) {
            String key = s.substring(i, i + 10);
            if(map.containsKey(key)){
                map.put(key, map.get(key) + 1);
            }else{
                map.put(key, 1);
            } 
        }

        for (Map.Entry<String, Integer> entry : map.entrySet()) {
            if (entry.getValue() > 1) {
                result.add(entry.getKey());
            }
        }
        return result;
    }
}