Coding

Find Non-Repeating Characters in a String

Asked at

Amazon

AtlassianCCognitree

1Give yourself 5 minutes

2Answer out loud, not in your head

3Then compare with the answer below

The problem

You are given a string s. Your task is to find all characters that appear exactly once in the string and return them in the order they appear in the original string.

Return the result as a comma-separated string of characters. If no non-repeating characters exist, return an empty string.

**Input: **s = "google" Output: "l,e" Explanation: Characters 'l' and 'e' appear only once and are in order.

Input: s = "yahoo" **Output: **"y,a,h" Explanation: Characters 'y', 'a', 'h' appear only once in order.

**Input: **s = "leetcode" Output: "l,t,c,d" Explanation: These are the characters that appear only once and appear in this order.

1 <= s.length <= 105
The string contains only lowercase English letters ('a' to 'z')

cpp

class Solution {
public:
    string findNonRepeatingCharacters(const string& s) {
        // Your code goes here
    }
};

java

class Solution {
    public String findNonRepeatingCharacters(String s) {
        // Your code goes here
    }
}

python

class Solution:
    def find_non_repeating_characters(self, s: str) -> str:
        # Your code goes here

javascript

class Solution {
    findNonRepeatingCharacters(s) {
        // Your code goes here
    }
}

csharp

class Solution
{
    public string FindNonRepeatingCharacters(string s)
    {
        // Your code goes here
    }
}

func FindNonRepeatingCharacters(s string) string {
}

Stuck? Show a way to structure it+

01Count every character in a first pass.
02Scan the original string again and collect characters whose count is one.
03Join the collected characters with commas without reordering them.
04Preserve encounter order by scanning the input again rather than the map.

Reference answer

First build a frequency map for every character in the string. Then scan the original string from left to right. Whenever the current character's frequency is exactly one, append it to an output list. Finally join that list with commas; if the list is empty, return an empty string. The second pass is necessary because iterating the frequency map would not reliably preserve the characters' positions in the original string. Time is O(n) and space is O(u), where u is the number of distinct characters. For a fixed ASCII alphabet, a fixed-size array can replace the map. Clarify whether matching is case-sensitive and whether a character means a byte, Unicode code point, or grapheme cluster; the basic algorithm remains the same, but representation affects iteration and keys.

Then expect these follow-ups

How would you return only the first non-repeating character in a stream?
Tests: streaming design
How would Unicode grapheme clusters affect the implementation?
Tests: text representation

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