187. Repeated DNA Sequences

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本文介绍了一种算法，用于从DNA分子中找出所有长度为10个字母且出现次数超过一次的重复序列。通过三种不同方法展示了如何利用HashSet和HashMap来高效解决这一问题。

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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"].

不同的方法显示了使用HashSet和HashMap的区别

import java.util.*;

/**
 * Created by happy on 17/4/29.
 */
public class RepeatedDNASequences {
    public List<String> findRepeatedDnaSequences(String s) {
        Set seen = new HashSet<>();
        Set repeated = new HashSet();
        for (int i = 0; i + 10 <= s.length(); i++) {
            String str = s.substring(i, i + 10);
            if (!seen.add(str)) {
                repeated.add(str);
            }
        }
        return new ArrayList<>(repeated);

    }

    public List<String> findRepeatedDnaSequencesII(String s) {
        List<String> res = new LinkedList<String>();
        HashMap map = new HashMap();
        Set repeated = new HashSet();
        for (int i = 0; i + 9 < s.length(); i++) {

            String str = s.substring(i, i + 10);
            if (map.containsKey(str)) {
                repeated.add(str);
            } else {
                map.put(str, 1);
            }
        }
        return new ArrayList<>(repeated);
    }

    public List<String> findRepeatedDnaSequencesIII(String s) {
        HashMap<String, Integer> map = new HashMap();
        List<String> res = new LinkedList<String>();
        Set repeated = new HashSet();
        for (int i = 0; i + 10 <= s.length(); i++) {
            String str = s.substring(i, i + 10);
            if (map.containsKey(str)) {
                if (map.get(str) == 1) {
                    map.replace(str, -1);
                    res.add(str);
                }
            } else {
                map.put(str, 1);
            }

        }
        return res;
    }
}