187. Repeated DNA Sequences

The DNA sequence is composed of a series of nucleotides abbreviated as ‘A’, ‘C’, ‘G’, and ‘T’.

• For example, “ACGAATTCCG” is a DNA sequence.
  When studying DNA, it is useful to identify repeated sequences within the DNA.

Given a string s that represents a DNA sequence, return all the 10-letter-long sequences (substrings) that occur more than once in a DNA molecule. You may return the answer in any order.

哈希表 + 滑动窗口 + 位操作。
将四个字符映射到四个二进制字符上。这样字符串就可以用20bit表示。
这样就可以用一个整数来表示这四个字符。
然后采用哈希表记录出现次数。

class Solution {
    static final int L = 10;
    public List<String> findRepeatedDnaSequences(String s) {
        List<String> ret = new ArrayList<String>();
        if(s.length() < L) return ret;
        HashMap<Integer, Integer> map = new HashMap<>();
        int left = 0;
        int right = 10;
        
        int[] bin = new int[26];
        bin['A'-'A'] = 0;
        bin['T'-'A'] = 1;
        bin['C'-'A'] = 2;
        bin['G'-'A'] = 3;
        
        int x = 0;
        for(int i = 0; i < L-1; i++){
            x = (x << 2) | bin[s.charAt(i)-'A'];
        }
        
        for(int i = 0; i <= s.length() - L; i++){
            x = ((x << 2) | bin[s.charAt(i + L -1)-'A']) & ((1 << L * 2) - 1);
            map.put(x, map.getOrDefault(x, 0)+1);
            if(map.get(x) == 2) ret.add(s.substring(i, i+L)); 
        }
        return ret;
    }
}

遍历将字字符串加入哈希表并记录出现次数，然后返回出现次数大于1的字符串。
注意在循环时就可以直接添加结果到列表，这样可以减少操作。

class Solution {
    public List<String> findRepeatedDnaSequences(String s) {
        List<String> ret = new ArrayList<String>();
        HashMap<String, Integer> map = new HashMap<>();
        int left = 0;
        int right = 10;
        
        while(right <= s.length()){
            String sub = s.substring(left, right);
            map.put(sub, map.getOrDefault(sub, 0)+1);
            if(map.get(sub) == 2) ret.add(sub); 
            left++;
            right++;
        }
        return ret;
    }
}