169. Majority Element

Given an array of size n, find the majority element. The majority element is the element that appears more than ⌊ n/2 ⌋ times.

You may assume that the array is non-empty and the majority element always exist in the array.

• 思路一

class Solution {
public:
    int majorityElement(vector<int>& nums) {

        sort(nums.begin(),nums.end());
        return nums[nums.size()/2];
    }
};

• 思路2

class Solution {
public:
    int majorityElement(vector<int>& nums) {

        map<int,int> count;
        for(int i=0; i<nums.size(); i++)
        {
            if(++count[nums[i]] > nums.size()/2)
                return nums[i];
        }
        return 0;
    }
};

• 思路3

摩尔投票法 Moore Voting

这种投票法先将第一个数字假设为众数，然后把计数器设为1，比较下一个数和此数是否相等，若相等则计数器加一，反之减一。然后看此时计数器的值，若为零，则将当前值设为候选众数。以此类推直到遍历完整个数组，当前候选众数即为该数组的众数。

class Solution {
public:
    int majorityElement(vector<int>& nums) {

    int idx=0, count=1;
    for(int i=1; i<nums.size(); i++)
    {
        nums[idx] == nums[i] ? ++count : --count;
        if(count == 0)
        {
            idx=i;
            count=1;
        }
    }
    return nums[idx];

    }
};