标签：return bre 8进制 new sed solution line str ble
题目：
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"].
链接： http://leetcode.com/problems/repeated-dna-sequences/
6/25/2017
好久没刷题了，这道题也是参考别人的答案。
48ms, 80%时间复杂度O(N*N*k)，k=10, 第一个N来自遍历数组，第二个N来自substring
注意第8行，结束的位置是i <= s.length() - 10，要包含最后一位。
1 public class Solution { 2 public List<String> findRepeatedDnaSequences(String s) { 3 List<String> res = new ArrayList<String>(); 4 if (s == null || s.length() < 10) { 5 return res; 6 } 7 Map<String, Integer> substringCount = new HashMap<String, Integer>(); 8 for (int i = 0; i <= s.length() - 10; i++) { 9 String substring = s.substring(i, i + 10); 10 if (substringCount.containsKey(substring)) { 11 int count = substringCount.get(substring); 12 if (count == 1) { 13 res.add(substring); 14 } 15 substringCount.put(substring, count + 1); 16 } else { 17 substringCount.put(substring, 1); 18 } 19 } 20 return res; 21 } 22 }
别人的答案：
类似rabin-karp，因为只有4个字符，所以每个字符用2位来表示（4^10 < 2^32），map里只需要比较数组而不是string，map的效率更高。链接里有解释
https://discuss.leetcode.com/topic/8894/clean-java-solution-hashmap-bits-manipulation
类似的，只不过用了8进制，链接里有解释，但是我稍微写详细一些。
t存的是所有10个字符的int hash值，这个值是通过这个算法里来计算的。注意有个ox3FFFFFFF，想明白了这个是只保留最后30位，为什么因为字符通过&7之后每个只保留3位2进制数，如果是10个字符的话正好是30位，可以消去10个字符之前的影响。
https://discuss.leetcode.com/topic/8487/i-did-it-in-10-lines-of-c
1 vector<string> findRepeatedDnaSequences(string s) { 2 unordered_map<int, int> m; 3 vector<string> r; 4 for (int t = 0, i = 0; i < s.size(); i++) 5 if (m[t = t << 3 & 0x3FFFFFFF | s[i] & 7]++ == 1) 6 r.push_back(s.substr(i - 9, 10)); 7 return r; 8 }
更多讨论
https://discuss.leetcode.com/category/195/repeated-dna-sequences
标签：return bre 8进制 new sed solution line str ble
原文地址：http://www.cnblogs.com/panini/p/7077834.html
