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# 0169. Majority Element

<https://leetcode.com/problems/majority-element>

## Description

Given an array `nums` of size `n`, return *the majority element*.

The majority element is the element that appears more than `⌊n / 2⌋` times. You may assume that the majority element always exists in the array.

**Example 1:**

```
**Input:** nums = [3,2,3]
**Output:** 3
```

**Example 2:**

```
**Input:** nums = [2,2,1,1,1,2,2]
**Output:** 2
```

**Constraints:**

* `n == nums.length`
* `1 <= n <= 5 * 104`
* `-231 <= nums[i] <= 231 - 1`

**Follow-up:** Could you solve the problem in linear time and in `O(1)` space?

## ac1: Boyer-Mooer

```java
class Solution {
    public int majorityElement(int[] nums) {
        // edge cases
        if (nums == null || nums.length == 0) throw new IllegalArgumentException("invalid input");

        // 1st pass
        int x = nums[0], cnt = 0;
        for (int n : nums) {
            if (n == x) {
                cnt++;
            } else if (cnt == 0) {
                x = n;
                cnt++;
            } else {
                cnt--;
            }
        }

        // asume exist, no 2nd pass check, return
        return x;
    }
}

/*
hashmap, O(n) time O(n) space
sorting, O(nlogn) time O(1/n) space
boyer-mooer, O(n) time O(1) space

*/
```