Java Solution for First Unique Character in a string using HashMap

  • 0

    Approach #1 using Two-pass hash table


    Check all the characters in the string one by one and insert in HashMap with character as the key and index as the value. If a key repeats, set its value to -1.


    Suppose we have a function intUniqChar(String S) . It returns the first unique character index.

    • Convert the string to char array and add all the characters to LinkedHashMap (to maintain the order).key is the character and value is the index.
      If a key already exists set its value to -1.
    • Add all the values to a LinkedList to maintain the order.
    • Find the first element from the list whose value is not equal to -1.
    • That value will be the index of first unique character in the string


    import java.util.Optional;
    class Solution {
        public int firstUniqChar(String s) {
            if (s.isEmpty())
                return -1;
            char[] charArray = s.toCharArray();
            Map<Character, Integer> map = new LinkedHashMap<>();
            // Adding the characters to a LinkedHashMap
            for (int i = 0; i < charArray.length; i++)
            List<Integer> values = new LinkedList<>();
            // Adding all the values in map to a LinkedList.
            // filtering the -1 indices and returning the first index.
            Optional<Integer> output =
                    .filter(x -> !x.equals(-1)).findFirst();
                return output.get();
            else return -1;

    Complexity Analysis

    • Time complexity : O(n)

    To verify if characters have duplicates, we need to read each character exactly once and it will lead to linear complexity which is O(n)

    • Space complexity : O(2m) (m <= lengthOfString)
      Since we insert all the unique characters only once in the HashMap which is m and also add all the values to LinkedList which is again m.
      So the space complexity is O(2m).

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