271 Encode and Decode Strings

1. Question

Design an algorithm to encodea list of stringstoa string. The encoded string is then sent over the network and is decoded back to the original list of strings.
Machine 1 (sender) has the function:
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string encode(vector<string> strs) {
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// ... your code
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return encoded_string;
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}
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Machine 2 (receiver) has the function:
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vector<string> decode(string s) {
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//... your code
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return strs;
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}
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So Machine 1 does:
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string encoded_string = encode(strs);
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and Machine 2 does:
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vector<string> strs2 = decode(encoded_string);
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strs2in Machine 2 should be the same asstrsin Machine 1.
Implement theencodeanddecodemethods.
Note:
  • The string may contain any possible characters out of 256 valid ascii characters. Your algorithm should be generalized enough to work on any possible characters.
  • Do not use class member/global/static variables to store states. Your encode and decode algorithms should be stateless.
  • Do not rely on any library method such as
    eval
    or serialize methods. You should implement your own encode/decode algorithm.

2. Implementation

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public class Codec {
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// Encodes a list of strings to a single string.
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public String encode(List<String> strs) {
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StringBuilder code = new StringBuilder();
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for (String str : strs) {
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code.append(str.length()).append("/").append(str);
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}
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return code.toString();
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}
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// Decodes a single string to a list of strings.
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public List<String> decode(String s) {
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List<String> strs = new ArrayList();
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int index = 0;
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while (index < s.length()) {
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int slashIndex = s.indexOf("/", index);
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int size = Integer.parseInt(s.substring(index, slashIndex));
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strs.add(s.substring(slashIndex + 1, slashIndex + size + 1));
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index = slashIndex + size + 1;
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}
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return strs;
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}
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}
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// Your Codec object will be instantiated and called as such:
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// Codec codec = new Codec();
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// codec.decode(codec.encode(strs));
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3. Time & Space Complexity

时间复杂度: encode:O(n), decode: O(n^2), n为decode string的长度,substring()需要O(n)的时间, 空间复杂度O(n)