LeetCode 187. Repeated DNA Sequences（哈希）

题目来源：https://leetcode.com/problems/repeated-dna-sequences/

问题描述

187. Repeated DNA Sequences

Medium

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"]

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题意

给定一个由ACTG组成的DNA序列，求长度为10且重复出现的子列（重复子列间允许重叠）

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思路

由于固定了子列长度，因此O(n)遍历字符串放到HashMap里统计出现次数即可。当然这个问题由于只有ACTG四种字符，因此可以将A/C/T/G二进制编码成00/01/10/11然后将10位子列编码成20位二进制对应的正整数，而无需调用Java String的hashCode

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代码

class Solution {
    public List findRepeatedDnaSequences(String s) {
        int n = s.length();
        if (n <= 10) {
            return Collections.emptyList();
        }
        HashMap map = new HashMap();
        List ans = new ArrayList();
        for (int i=0; i<=n-10; ++i) {
            String str10 = s.substring(i, i+10);
            if (map.containsKey(str10)) {
                if (map.get(str10) == 1) {
                    ans.add(str10);
                }
                map.put(str10, map.get(str10) + 1);
            } else {
                map.put(str10, 1);
            }
        }
        return ans;
    }
}