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

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

*/