Efficient way to split a string in one pass to produce a key value pair and check corner case if key is empty

Question

Hi guys i have a string "key={value}&key2={value2}&key3={value3}" and have to produce a key-value pair of the string by splitting & and =. Also i have to watch out for corner cases such that if a key isn't present do not add it to the hash map but if a value is missing the key can be added with an empty value. One way is to first split the string by & and finally by =. So this can be accomplished but having two passes. But i am attempting to accomplish this in one pass.

First idea: I came up with one pass using the split function using but it didn't account for corner cases.

public Map<String, String> splitString(String s) {
    // String s represents ParamAndMacro
    String[] split = s.split("=|\\&");
    int length = split.length;
    Map<String, String> maps = new HashMap<>();
    for (int i=0; i<length; i+=2){
          maps.put(split[i], split[i+1]);
     } 
    return maps;
}

Second Idea: One pass with certain conditions and using stringbuilder:

public Map<String, String> splitString(String s){
 Map<String, String> maps = new HashMap<>();
 int length = s.length();
 StringBuilder sb = new StringBuilder();
 String key = "";
 String value = "";

char prev = ' ';
for (int i=0; i<length; i++) {
    char a = s.charAt(i);
    if ( a != '=' && a != '&' && prev != a) {
           sb.append(a);        
      }
    //finds a key
    else if( a == '=' && prev != a && prev != '&') {
          key = sb.toString();
          sb = new StringBuilder();
      }
    // finds a value and also ensuring a key is not empty
    else if (a=='&' && prev != a && prev != '=') {
          value = sb.toString();
          sb = new StringBuilder();
          maps.put(key, value);
            }
    // checks corner case to see if a value is empty
    else if (prev=='=' && a == '&') {
          value = "";
          maps.put(key, value);
          sb = new StringBuilder();
    }
   //how to check if a key is missing and skip to other pair

            prev = a;
    }
    return maps;
}

My question is my code efficient in terms of splitting and using stringbuilder. Please feel free to improve or give suggestions.