Determine if String Halves Are Alike

Determine if String Halves Are Alike - Problem

String Counting Easy

You are given a string s of even length. Split this string into two halves of equal lengths, and let a be the first half and b be the second half.

Two strings are alike if they have the same number of vowels ('a', 'e', 'i', 'o', 'u', 'A', 'E', 'I', 'O', 'U'). Notice that s contains uppercase and lowercase letters.

Return true if a and b are alike. Otherwise, return false.

Input & Output

Example 1 — Basic Case

$ Input: s = "AbCdEfGh"

› Output: true

💡 Note: First half: "AbCd" has 1 vowel (A). Second half: "EfGh" has 1 vowel (E). Both halves have equal vowels, so return true.

Example 2 — Unequal Vowels

$ Input: s = "book"

› Output: true

💡 Note: First half: "bo" has 1 vowel (o). Second half: "ok" has 1 vowel (o). Both halves have equal vowels, so return true.

Example 3 — No Vowels

$ Input: s = "textbook"

› Output: false

💡 Note: First half: "text" has 1 vowel (e). Second half: "book" has 2 vowels (o, o). 1 ≠ 2, so return false.

Constraints

2 ≤ s.length ≤ 1000
s.length is even
s consists of uppercase and lowercase letters

Visualization

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Asked in

f Facebook 15 a Amazon 12

The key insight is to count vowels in both halves and compare. The optimal approach uses a single pass with a balance counter: +1 for first half vowels, -1 for second half vowels. If the final balance is 0, the halves are alike. Time: O(n), Space: O(1).

Common Approaches

✓ Single Pass with Balance Counter

⏱️ Time: O(n) Space: O(1)

Instead of counting vowels in each half separately, we use a balance counter. Add 1 for each vowel in the first half and subtract 1 for each vowel in the second half. If the final balance is 0, the halves have equal vowels.

Two Separate Counts

⏱️ Time: O(n) Space: O(1)

Split the string into two halves and count vowels in each half separately. This is the most straightforward approach that directly follows the problem description.

Single Pass with Balance Counter — Algorithm Steps

Initialize balance counter to 0
For each character, check if it's a vowel
If vowel in first half: add 1, if in second half: subtract 1
Return true if final balance is 0

Visualization

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Step-by-Step Walkthrough

Initialize

Start with balance = 0

Process

Add 1 for first half vowels, subtract 1 for second half vowels

Result

If balance = 0, halves are alike

Code -

solution.c — C

#include <stdio.h>
#include <string.h>
#include <stdbool.h>

bool isVowel(char c) {
    return c == 'a' || c == 'e' || c == 'i' || c == 'o' || c == 'u' ||
           c == 'A' || c == 'E' || c == 'I' || c == 'O' || c == 'U';
}

bool solution(char* s) {
    int n = strlen(s);
    int mid = n / 2;
    int balance = 0;
    
    for (int i = 0; i < n; i++) {
        if (isVowel(s[i])) {
            if (i < mid) {
                balance += 1;  // First half vowel
            } else {
                balance -= 1;  // Second half vowel
            }
        }
    }
    
    return balance == 0;
}

int main() {
    char s[1001];
    fgets(s, sizeof(s), stdin);
    s[strcspn(s, "\n")] = 0;
    
    bool result = solution(s);
    printf(result ? "true\n" : "false\n");
    return 0;
}

Time & Space Complexity

Time Complexity

⏱️

O(n)

Single pass through the string, checking each character once

✓ Linear Growth

Space Complexity

O(1)

Only using a balance counter variable and vowel set

✓ Linear Space

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Medium Frequency

~8 min Avg. Time

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Input & Output

Constraints

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Related Problems

Common Approaches

Single Pass with Balance Counter — Algorithm Steps

Visualization

Code -

Time & Space Complexity

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