LeetCode 229 : Majority Element II

Given an integer array of size n, find all elements that appear more than ⌊ n/3 ⌋ times. The algorithm should run in linear time and in O(1) space.

如果没有这时间空间要求，就跟 169 Majority Element 那题一样，用hash table， 或者 sort 都能很方便的实现。但是加上这些时间空间要求后，一开始想不出思路。

说明没有彻底理解169题中那种 O(n) 的方法。这题的解法和那种方法相似。

public class Solution {
    public List<Integer> majorityElement(int[] nums) {
        List<Integer> list = new ArrayList<>();
        if (nums == null)
            return null;
        if (nums.length < 3) {
             for (int i = 0; i < nums.length; i++) {
                if (!list.contains(nums[i]))
                    list.add(nums[i]);
            }
            return list;
        }
            
        int val1 = nums[0], val2 = nums[1], count1 = 0, count2 = 0;
        for (int i = 0; i < nums.length; i++) {
            if (val1 == nums[i]) count1++;
            else if (val2 == nums[i]) count2++;
            else if (count1 == 0) {
                val1 = nums[i];
                count1++;
            } else if (count2 == 0) {
                val2 = nums[i];
                count2++;
            } else {
                count1--;
                count2--;
            } 
        }
        check(nums, val1, val2, list);
        return list;
    }

    public void check(int[] nums, int val1, int val2, List<Integer> list) {
        int count1 = 0, count2 = 0;
        for (int i = 0; i < nums.length; i++) {
            if (nums[i] == val1)
                count1++;
            else if (nums[i] == val2)
                count2++;
        }
        if (count1 > nums.length / 3)
            list.add(val1);
        if (count2 > nums.length / 3)
            list.add(val2);
    }
}

2015-10-21