LeetCode Problem 169: Majority Element查找多数元素

描述：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1：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)

 1 class Solution {
 2 public:
 3     int majorityElement(vector<int> &num) {
 4 
 5         int elem = 0;
 6         int count = 0;
 7         
 8         for(int i = 0; i < num.size(); i++)  {
 9             
10             if(count == 0)  {
11                 elem = num[i];
12                 count = 1;
13             }
14             else    {
15                 if(elem == num[i])
16                     count++;
17                 else
18                     count--;
19             }
20             
21         }
22         return elem;
23     }
　　};

思路2：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

 1 class Solution {
 2 public:
 3     int majorityElement(vector<int> &num) {
 4 
 5         int count = 0;
 6         
 7         for(;;) {
 8             if(num.size() == 1)
 9                 return num[0];
10             else    {
11                 int i = rand() % (num.size() - 1);
12                 for(int j = 0; j < num.size(); j++) {
13                     if(num[j] == num[i])
14                         count++;
15                 }
16                 if(count > (num.size() / 2))
17                     return num[i];
18                 else    {
19                     count = 0;
20                     continue;
21                 }
22             }
23         }
24     }
　　};

附LeetCode建议解决方案：

LeetCode Problem 169: Majority Element查找多数元素

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