229. Majority Element II

Given an integer array of size n, find all elements that appear more than ⌊ n/3 ⌋ times.

Note: The algorithm should run in linear time and in O(1) space.

Example 1:

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

Example 2:

Input: [1,1,1,3,3,2,2,2]
Output: [1,2]

思路：由於要求算法的時間複雜度爲O（N），並且空間複雜度爲O（1），那麼哈希表，排序等算法均不能考慮

但我們注意到題目要求出現的衆數必須出現n/3次以上，易知這樣的數在一個數組中出現最多隻有兩個

由此，按照摩爾投票的模板，我們容易寫出如下代碼

class Solution {
    public List<Integer> majorityElement(int[] nums) {
        List<Integer> res = new ArrayList<>();
        if(nums == null || nums.length == 0)
            return res;
        //定義兩個候選人
        int candidateA = nums[0];
        int candidateB = nums[0];
        int countA = 0;
        int countB = 0;
        for(int num:nums) {
            if(num == candidateA) {
                countA++;
                continue;
            }
            if(num == candidateB) {
                countB++;
                continue;
            }
            if(countA == 0) {
                candidateA = num;
                countA = 1;
                continue;
            }
            if(countB == 0) {
                candidateB = num;
                countB = 1;
                continue;
            }
            countA--;
            countB--;
        }
        countA = 0;
        countB = 0;
        for(int num:nums) {
            if(num == candidateA) {
                countA++;
            } else if(num == candidateB) {
                countB++;
            }
        }
        if(countA > nums.length/3) {
            res.add(candidateA);
        }
        if(countB > nums.length/3) {
            res.add(candidateB);
        }
        return res;
    }
}