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.

class Solution {
public:
    vector<int> majorityElement(vector<int>& nums) {
        vector<int> res;
        if(nums.size() ==0) 
             return nums;  
        if(nums.size()==1) {  
            return nums;  
        }  
          
        int m1 = nums[0];  
        int m2 = 0;  
          
        int c1 = 1;  
        int c2 = 0;  
          
        for(int i=1; i<nums.size(); i++) {  
            int x = nums[i];  
            if(x==m1) ++c1;  
            else if(x==m2) ++c2;  
            else if(c1==0) {  
                m1 = x;  
                c1 = 1;  
            } else if(c2==0) {  
                m2 = x;  
                c2 = 1;  
            } else {  
                --c1; --c2;  
            }  
        }  
        c1 = 0; c2 = 0;  
        for(int i=0; i<nums.size(); i++) {  
            if(m1 == nums[i]) ++c1;  
            else if(m2 == nums[i]) ++c2;  
        }  
        if(c1>nums.size()/3) res.push_back(m1);  
        if(c2>nums.size()/3) res.push_back(m2);  
        return res;
    }
};