[Algorithms with Rust] #2 firstNotRepeatingCharacter - CodeSignal

firstNotRepeatingCharacter

Given a string s consisting of small English letters, find and return the first instance of a non-repeating character in it. If there is no such character, return '_'.

Example

• For s = "abacabad", the output should be firstNotRepeatingCharacter(s) = 'c'.

  There are 2 non-repeating characters in the string: 'c' and 'd'. Return c since it appears in the string first.

• For s = "abacabaabacaba", the output should be firstNotRepeatingCharacter(s) = '_'.

  There are no characters in this string that do not repeat.

Input/Output

• [execution time limit] 2 seconds (rs)

• [input] string s

  A string that contains only lowercase English letters.

  Guaranteed constraints:

  1 ≤ s.length ≤ 10**5,

• [output] char

  The first non-repeating character in s, or '_' if there are no characters that do not repeat.

Solution

use std::collections::HashMap;

fn firstNotRepeatingCharacter(s: String) -> char {
    let mut m = HashMap::new();
    for i in s.chars() {
        if let Some(&c) = m.get(&i) {
            m.insert(i, c + 1);
        } else {
            m.insert(i, 1);
        }
    }

    for i in s.chars() {
        if let Some(&1) = m.get(&i) {
            return i;
        }
    }

    '_'
}