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.

Hint:

1. How many majority elements could it possibly have?
2. Do you have a better hint? Suggest it!

这道题考的是摩尔投票法。具体可以参考GrandYang的解释。代码如下

public IList<int> MajorityElement(int[] nums) {
        int m = 0;
        int n =0;
        int cm =0;
        int cn =0;
        foreach(var num in nums)
        {
            if(m == num) cm++;
            else if(n == num) cn++;
            else if(cm ==0 )
            {
                cm=1;
                m =num;
            }
            else if(cn==0)
            {
                cn=1;
                n = num;
            }
            else
            {
                cm--;
                cn--;
            }
        }
        cm =0;
        cn=0;
        foreach(var num in nums)
        {
            if(m == num) cm++;
            else if(n == num) cn++;
        }
        var res = new List<int>();
        if(cm > nums.Count()/3) res.Add(m);
        if(cn > nums.Count()/3) res.Add(n);
        return res;
    }

注意几个corner case

[0,0,0]， 这个要求第二遍循环的时候必须是if else， 如果不是的话会return [0,0].

[0,1] 还需要注意一点是最后cm cn是要大于而不是大于等于。不然这个输出回事[0,1]