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.

方法一：将数组排序，元素个数多于一半必在数组中间

class Solution {
public:
int majorityElement(vector<int>& nums) {
if(nums.empty())
return 0;
sort(nums.begin(),nums.end());
return nums[nums.size()/2];
}
};

方法二：利用哈希表统计各元素个数

class Solution {
public:
int majorityElement(vector<int>& nums) {
unordered_map<int,int> map;
for(int i=0;i<nums.size();i++)
{
map[nums[i]]++;
}
for(auto j:map)
{
if(j.second>nums.size()/2)
return j.first;
}
}
};

方法三：（网上摘的）投票算法，每次遇到不同，票数减一，最后剩下的就是候选人

class Solution {
public:
int majorityElement(vector<int>& nums) {
int count=0;
int res=0;
for(int i=0;i<nums.size();i++)
{
if(count==0)
{
res=nums[i];
count=1;
}
else
{
if(res==nums[i])
count++;
else
count--;
}
}
return res;
}
};

class Solution {
public:
int majorityElement(vector<int>& nums) {
int cnt = 1;
int num = nums[0];
for (int i = 1; i < nums.size(); i++){
if (nums[i] == num)
cnt++;
else{
cnt--;
if (cnt == 0){
num = nums[i];
cnt = 1;
}
}
}
return num;
}
};