The special characters and their entities for HTML are:

• Quotation Mark: the entity is " and symbol character is ".
• Single Quote Mark: the entity is ' and symbol character is '.
• Ampersand: the entity is & and symbol character is &.
• Greater Than Sign: the entity is > and symbol character is >.
• Less Than Sign: the entity is < and symbol character is <.
• Slash: the entity is ⁄ and symbol character is /.

Given the input text string to the HTML parser, you have to implement the entity parser.

Return the text after replacing the entities by the special characters.

Example 1:

Input: text = "& is an HTML entity but &ambassador; is not."
Output: "& is an HTML entity but &ambassador; is not."
Explanation: The parser will replace the & entity by &

Example 2:

Input: text = "and I quote: "...""
Output: "and I quote: \"...\""

Example 3:

Input: text = "Stay home! Practice on Leetcode :)"
Output: "Stay home! Practice on Leetcode :)"

Example 4:

Input: text = "x > y && x < y is always false"
Output: "x > y && x < y is always false"

Example 5:

Input: text = "leetcode.com⁄problemset⁄all"
Output: "leetcode.com/problemset/all"

Constraints:

• 1 <= text.length <= 10^5
• The string may contain any possible characters out of all the 256 ASCII characters.

Solution: Simulation

Time complexity: O(n)
Space complexity: O(n)

// Author: Huahua
class Solution {
public:
  string entityParser(string text) {    
    map<string, string> m{
      {"&quot;", "\""}, {"&apos;", "'"}, {"&amp;", "&"}, 
      {"&gt;", ">"}, {"&lt;", "<"}, {"&frasl;", "/"}};
    string ans;
    string buf;
    for (char c : text) {
      buf += c;
      if (buf.back() != ';') continue;
      const int l = buf.size();
      for (const auto& [k, v] : m) {
        const int kl = k.length();
        if (l >= kl && buf.substr(l - kl) == k) {
          ans += buf.substr(0, l - kl) + v;
          buf.clear();
          break;
        }            
      }      
    }    
    return ans + buf;
  }
};

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