LeetCode: Majority Element

Challenge Description

Given an array of size n, find the majority element. The majority element is the element that appears more than ⌊ n/2 ⌋ times.

You may assume that the array is non-empty and the majority element always exist in the array.

Example 1:

Input: [3,2,3]
Output: 3

Example 2:

Input: [2,2,1,1,1,2,2] 
Output: 2

Naive Solution

Iterate over all the elements in array, use a unordered_map to store the counts of current element, when one element’s count > size/2, it’s OK to break the loop and return the element.

Time complexity: $O(N)$.

class Solution {
public:
    int majorityElement(vector<int>& nums) {
        unordered_map<int, int> counts;
        unordered_map<int, int>::iterator it;
        for(int i=0; i < nums.size(); i++) {
            int t = nums[i];
            it = counts.find(t);
            if(it == counts.end()) 
                counts[t] = 1;
            else 
                counts[t] += 1;
            if(counts[t] > nums.size() / 2)
                return t;
        }
        return 0;
    }
};

Approach with sorting

If we sort the array, the element at index size/2 will be the answer.

Time complexity: $O(NlogN)$.

class Solution {
public:
    int majorityElement(vector<int>& nums) {
        sort(nums.begin(), nums.end());
        return nums[nums.size()/2];
    }
};

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