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Majority Number

LintCode-46. Majority Number

Given an array of integers, the majority number is the number that occurs more than half of the size of the array. Find it.

Example

Given [1, 1, 1, 1, 2, 2, 2], return 1

Challenge

O(n) time and O(1) extra space

This problem could be solved using Boyer–Moore majority vote algorithm.

public class Solution {  
    /**
     * @param nums: a list of integers
     * @return: find a  majority number
     */
    public int majorityNumber(ArrayList<Integer> nums) {
        if (nums == null || nums.size() == 0) {
            return -1;
        }

        int candidate = 0, counter = 0;

        for (int i : nums) {
            if (counter == 0) {
                candidate = i;
                counter = 1;
            } else if (i == candidate) {
                counter++;
            } else if (i != candidate) {
                counter--;
            }
        }

        return candidate;
    }
}

Hope this helps,
Michael