LeetCode #1189 — EASY

Maximum Number of Balloons

Build confidence with an intuition-first walkthrough focused on hash map fundamentals.

The Problem

Problem Statement

Given a string text, you want to use the characters of text to form as many instances of the word "balloon" as possible.

You can use each character in text at most once. Return the maximum number of instances that can be formed.

Example 1:

Input: text = "nlaebolko"
Output: 1

Example 2:

Input: text = "loonbalxballpoon"
Output: 2

Example 3:

Input: text = "leetcode"
Output: 0

Constraints:

1 <= text.length <= 10⁴
text consists of lower case English letters only.

Note: This question is the same as 2287: Rearrange Characters to Make Target String.

Step 01

Brute Force Baseline

Problem summary: Given a string text, you want to use the characters of text to form as many instances of the word "balloon" as possible. You can use each character in text at most once. Return the maximum number of instances that can be formed.

Baseline thinking

Start with the most direct exhaustive search. That gives a correctness anchor before optimizing.

Pattern signal: Hash Map

Example 1

"nlaebolko"

Example 2

"loonbalxballpoon"

Example 3

"leetcode"

Step 02

Core Insight

What unlocks the optimal approach

Count the frequency of letters in the given string.
Find the letter than can make the minimum number of instances of the word "balloon".

Interview move: turn each hint into an invariant you can check after every iteration/recursion step.

Step 03

Algorithm Walkthrough

Iteration Checklist

Define state (indices, window, stack, map, DP cell, or recursion frame).
Apply one transition step and update the invariant.
Record answer candidate when condition is met.
Continue until all input is consumed.

Use the first example testcase as your mental trace to verify each transition.

Step 04

Edge Cases

Minimum Input

Single element / shortest valid input

Validate boundary behavior before entering the main loop or recursion.

Duplicates & Repeats

Repeated values / repeated states

Decide whether duplicates should be merged, skipped, or counted explicitly.

Extreme Constraints

Upper-end input sizes

Re-check complexity target against constraints to avoid time-limit issues.

Invalid / Corner Shape

Empty collections, zeros, or disconnected structures

Handle special-case structure before the core algorithm path.

Step 05

Full Annotated Code

Source-backed implementations are provided below for direct study and interview prep.

// Accepted solution for LeetCode #1189: Maximum Number of Balloons
class Solution {
    public int maxNumberOfBalloons(String text) {
        int[] cnt = new int[26];
        for (int i = 0; i < text.length(); ++i) {
            ++cnt[text.charAt(i) - 'a'];
        }
        cnt['l' - 'a'] >>= 1;
        cnt['o' - 'a'] >>= 1;
        int ans = 1 << 30;
        for (char c : "balon".toCharArray()) {
            ans = Math.min(ans, cnt[c - 'a']);
        }
        return ans;
    }
}

// Accepted solution for LeetCode #1189: Maximum Number of Balloons
func maxNumberOfBalloons(text string) int {
	cnt := [26]int{}
	for _, c := range text {
		cnt[c-'a']++
	}
	cnt['l'-'a'] >>= 1
	cnt['o'-'a'] >>= 1
	ans := 1 << 30
	for _, c := range "balon" {
		if x := cnt[c-'a']; ans > x {
			ans = x
		}
	}
	return ans
}

# Accepted solution for LeetCode #1189: Maximum Number of Balloons
class Solution:
    def maxNumberOfBalloons(self, text: str) -> int:
        cnt = Counter(text)
        cnt['o'] >>= 1
        cnt['l'] >>= 1
        return min(cnt[c] for c in 'balon')

// Accepted solution for LeetCode #1189: Maximum Number of Balloons
impl Solution {
    pub fn max_number_of_balloons(text: String) -> i32 {
        let mut arr = [0; 5];
        for c in text.chars() {
            match c {
                'b' => {
                    arr[0] += 1;
                }
                'a' => {
                    arr[1] += 1;
                }
                'l' => {
                    arr[2] += 1;
                }
                'o' => {
                    arr[3] += 1;
                }
                'n' => {
                    arr[4] += 1;
                }
                _ => {}
            }
        }
        arr[2] /= 2;
        arr[3] /= 2;
        let mut res = i32::MAX;
        for num in arr {
            res = res.min(num);
        }
        res
    }
}

// Accepted solution for LeetCode #1189: Maximum Number of Balloons
function maxNumberOfBalloons(text: string): number {
    const cnt = new Array(26).fill(0);
    for (const c of text) {
        cnt[c.charCodeAt(0) - 97]++;
    }
    return Math.min(cnt[0], cnt[1], cnt[11] >> 1, cnt[14] >> 1, cnt[13]);
}

Step 06

Interactive Study Demo

Use this to step through a reusable interview workflow for this problem.

Custom checkpoints (one per line)

Press Step or Run All to begin.

Step 07

Complexity Analysis

Time

O(n)

Space

O(C)

Approach Breakdown

BRUTE FORCE

O(n²) time

O(1) space

For each element, scan the rest of the array looking for a match. Two nested loops give n × (n−1)/2 comparisons = O(n²). No extra space since we only use loop indices.

HASH MAP

O(n) time

O(n) space

One pass through the input, performing O(1) hash map lookups and insertions at each step. The hash map may store up to n entries in the worst case. This is the classic space-for-time tradeoff: O(n) extra memory eliminates an inner loop.

Shortcut: Need to check “have I seen X before?” → hash map → O(n) time, O(n) space.

Coach Notes

Common Mistakes

Review these before coding to avoid predictable interview regressions.

Mutating counts without cleanup

Wrong move: Zero-count keys stay in map and break distinct/count constraints.

Usually fails on: Window/map size checks are consistently off by one.

Fix: Delete keys when count reaches zero.