【leetcode】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.

Credits:
Special thanks to @ts for adding this problem and creating all test cases.

 

Moore voting algorithm

每找出两个不同的element，就成对删除即count--，最终剩下的一定就是所求的。时间复杂度：O(n)

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

 

随机挑选一个元素，检查是否是多数元素。时间复杂度：Average：O(n)。期望查找次数 <2

class Solution {
 public:
     int majorityElement(vector<int> &num) {
 
         int count = 0;
         
         for(;;) {
             if(num.size() == 1)
                 return num[0];
             else    {
                 int i = rand() % (num.size() - 1);
                 for(int j = 0; j < num.size(); j++) {
                     if(num[j] == num[i])
                        count++;
                 }
                 if(count > (num.size() / 2))
                     return num[i];
                 else    {
                     count = 0;
                     continue;
                 }
             }
         }
     }
 };