First Non-Repeating Character in a Character Stream

1. Problem Description

Implement a function that finds the first character that appears only once in a character stream. For example, after reading only the first two characters “go”, the first character that appears once is “g”. After reading the first six characters “google”, the first character that appears once is “l”.

2. Approach

• Use a map for counting + a list to preserve order

3. Implementation

package main

var countMap = make(map[byte]int, 0)
var str = make([]byte, 0)

func Insert(ch byte) {
	countMap[ch]++
	str = append(str, ch)
}

func FirstAppearingOnce() byte {
	for _, s := range str {
		if countMap[s] == 1 {
			return s
		}
	}

	return '#'
}

4. References

• First Non-Repeating Character in a Character Stream - NowCoder
• 387. First Unique Character in a String - LeetCode