[LeetCode] Majority Element II

Majority Element II

Given an integer array of size n, find all elements that appear more than ⌊ n/3 ⌋ times. The algorithm should run in linear time and in O(1) space.

Hint:

1. How many majority elements could it possibly have?

跟出现超过一半的数差不多，出现超过1/3的数最多有2个，扫描一遍数组，分别计数数组中的元素，当出现3个不同的元素时，把这3个元素都抛弃掉，如果有符合要求的数，那么它一定在最后剩下的数中，再分别判断一下剩下的数就能得到答案。

 1 class Solution {
 2 public:
 3     vector<int> majorityElement(vector<int>& nums) {
 4         int k = 3;
 5         vector<int> val(k), cnt(k, 0);
 6         for (int i = 0; i < nums.size(); ++i) {
 7             bool flag = true;
 8             for (int j = 0; j < k; ++j)  {
 9                 if (val[j] == nums[i] && cnt[j] > 0) {
10                     val[j] = nums[i];
11                     ++cnt[j];
12                     flag = false;
13                     break;
14                 }
15             }
16             if (flag) {
17                 for (int j = 0; j < k; ++j) {
18                     if (cnt[j] == 0) {
19                         val[j] = nums[i];
20                         ++cnt[j];
21                         break;
22                     }
23                 }
24             }
25             if (cnt[k - 1] == 1) {
26                 for (int kk = 0; kk < k; ++kk) {
27                     --cnt[kk];
28                 }
29             }
30         }
31         vector<int> res;
32         for (int i = 0; i < k; ++i) if (cnt[i] > 0) {
33                 int c = 0;
34                 for (auto n : nums) if (val[i] == n) ++c;
35                 if (c > nums.size() / k) res.push_back(val[i]);
36         }
37         return res;
38     }
39 };