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Question

Formatted question description: https://leetcode.ca/all/387.html

Given a string s, find the first non-repeating character in it and return its index. If it does not exist, return -1.

 

Example 1:

Input: s = "leetcode"
Output: 0

Example 2:

Input: s = "loveleetcode"
Output: 2

Example 3:

Input: s = "aabb"
Output: -1

 

Constraints:

  • 1 <= s.length <= 105
  • s consists of only lowercase English letters.

Algorithm

The hash table builds a mapping between each character and its number of occurrences, and then traverses the string in order, finds the first character with a number of occurrences of 1, and returns its position.

Code

  • 
public class First_Unique_Character_in_a_String {

    public static void main (String[] args) {
        First_Unique_Character_in_a_String out = new First_Unique_Character_in_a_String();
        Solution s = out.new Solution();

        System.out.println(s.firstUniqChar("loveleetcode"));
    }

    class Solution {
        public int firstUniqChar(String s) {

            if (s == null || s.length() == 0) {
                return -1;
            }

            // unique[i] meaning index i has no duplicate
            int[] count = new int[256];

            for (char each: s.toCharArray()) {
                count[each] += 1;
            }

            for (int i = 0; i < s.length(); i++) {
                if (count[s.charAt(i)] == 1) {
                    return i;
                }
            }

            return -1;
        }
    }
}

############

class Solution {
    public int firstUniqChar(String s) {
        int[] cnt = new int[26];
        int n = s.length();
        for (int i = 0; i < n; ++i) {
            ++cnt[s.charAt(i) - 'a'];
        }
        for (int i = 0; i < n; ++i) {
            if (cnt[s.charAt(i) - 'a'] == 1) {
                return i;
            }
        }
        return -1;
    }
}

  • // OJ: https://leetcode.com/problems/first-unique-character-in-a-string/
// Time: O(S)
// Space: O(1)
class Solution {
public:
    int firstUniqChar(string s) {
        int cnt[26] = {0};
        for (char c : s) cnt[c - 'a']++;
        for (int i = 0; i < s.size(); ++i) {
            if (cnt[s[i] - 'a'] == 1) return i;
        }
        return -1;
    }
};

  • class Solution:
    def firstUniqChar(self, s: str) -> int:
        counter = Counter(s)
        for i, c in enumerate(s):
            if counter[c] == 1:
                return i
        return -1

############

class Solution(object):
  def firstUniqChar(self, s):
    """
    :type s: str
    :rtype: int
    """
    letters = [0] * 26
    for c in s:
      ci = ord(c) - ord('a')
      letters[ci] += 1
    for i in range(0, len(s)):
      ci = ord(s[i]) - ord('a')
      if letters[ci] == 1:
        return i
    return -1


  • func firstUniqChar(s string) int {
	cnt := [26]int{}
	for _, c := range s {
		cnt[c-'a']++
	}
	for i, c := range s {
		if cnt[c-'a'] == 1 {
			return i
		}
	}
	return -1
}

  • function firstUniqChar(s: string): number {
    const cnt = new Array(26).fill(0);
    for (const c of s) {
        cnt[c.charCodeAt(0) - 97]++;
    }
    for (let i = 0; i < s.length; i++) {
        if (cnt[s.charCodeAt(i) - 97] === 1) {
            return i;
        }
    }
    return -1;
}


  • /**
 * @param {string} s
 * @return {number}
 */
var firstUniqChar = function (s) {
    const cnt = new Array(26).fill(0);
    for (const c of s) {
        ++cnt[c.charCodeAt() - 'a'.charCodeAt()];
    }
    for (let i = 0; i < s.length; ++i) {
        if (cnt[s[i].charCodeAt() - 'a'.charCodeAt()] === 1) {
            return i;
        }
    }
    return -1;
};


  • class Solution {
    /**
     * @param String $s
     * @return Integer
     */
    function firstUniqChar($s) {
        for ($i = 0; $i < strlen($s); $i++) {
            $hashtable[$s[$i]]++;
        }
        for ($i = 0; $i < strlen($s); $i++) {
            if ($hashtable[$s[$i]] == 1) {
                return $i;
            }
        }
        return -1;
    }
}

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