leetcode 169.Majority Element

这道题虽然简单，但适合用来练习各种解法。

用哈希表解决

class Solution {
public:
    int majorityElement(vector<int>& nums) {
        unordered_map<int,int> count;
        int majority = 0;
        int cnt = 0;
        for(auto num:nums){
            count[num]++;
            if(count[num] > cnt){
                cnt = count[num];
                majority = num;
            }
        }
        return majority;
    }
};

先排序再取nums[n/2]

class Solution {
public:
    int majorityElement(vector<int>& nums) {
        int len = nums.size();
        srand(time(0));
        quick_sort(nums,0,len-1);
        return nums[len/2];
    }

    int partition(vector<int>& nums,int left,int right){
        if(left >= right)
            return left;
        int random = rand()%(right - left +1);
        swap(nums[left],nums[left + random]);
        int pivot = nums[left];
        while(left < right){
            while(left<right && pivot < nums[right]) right--;
            nums[left] = nums[right];  
            while(left<right && nums[left]<=pivot) left++;
            nums[right] = nums[left];
        }
        nums[left] = pivot;
        return left;
    }
    void quick_sort(vector<int>& nums,int left,int right){
        if(left>=right) return;
        int pivot_pos = partition(nums,left,right);
        quick_sort(nums,left,pivot_pos-1);
        quick_sort(nums,pivot_pos+1,right);
    }
};

随机化方法

class Solution {
public:
    int majorityElement(vector<int>& nums) {
        srand(0);
        int len = nums.size();
        int random = 0; 
        int cnt = 0;
        while(true){
            random = rand()%len;
            cnt = 0;
            for(auto num:nums){
                if(num == nums[random]){
                    cnt++;
                    if(cnt >len/2)
                        return num;
                }
            }
        }
    }
};