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10. Majority Element

easyAsked at N26

Find the element that appears more than n/2 times in an array. N26 frames this as detecting the dominant currency in a multi-currency batch.

By Alex Chen, Founder, InterviewChamp.AI · Last verified

Problem

Given an array nums of size n, return the majority element which always appears more than floor(n/2) times. You may assume the majority element always exists.

Constraints

  • n == nums.length
  • 1 <= n <= 5 * 10^4
  • Majority element is guaranteed to exist

Examples

Example 1

Input
nums=[3,2,3]
Output
3

Example 2

Input
nums=[2,2,1,1,1,2,2]
Output
2

Approaches

1. Hash map count

Count each element, return the one whose count exceeds n/2.

Time
O(n)
Space
O(n)
const m = new Map();
for (const x of nums) m.set(x, (m.get(x)||0)+1);
for (const [k,v] of m) if (v > nums.length/2) return k;

Tradeoff:

2. Boyer-Moore voting

Maintain a candidate and a count; flip candidate when count hits zero. The true majority survives every cancellation pass.

Time
O(n)
Space
O(1)
function majorityElement(nums) {
  let cand = 0, count = 0;
  for (const n of nums) {
    if (count === 0) cand = n;
    count += (n === cand) ? 1 : -1;
  }
  return cand;
}

Tradeoff:

N26-specific tips

N26 likes the Boyer-Moore answer because O(1) memory matches how their stream processors flag the dominant currency in a multi-currency settlement window without buffering.

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Output

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