[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。

class Solution {
public:
    vector<int> majorityElement(vector<int>& nums) {
        int k = 3;
        vector<int> val(k), cnt(k, 0);
        for (int i = 0; i < nums.size(); ++i) {
            bool flag = true;
            for (int j = 0; j < k; ++j)  {
                if (val[j] == nums[i] && cnt[j] > 0) {
                    val[j] = nums[i];
                    ++cnt[j];
                    flag = false;
                    break;
                }
            }
            if (flag) {
                for (int j = 0; j < k; ++j) {
                    if (cnt[j] == 0) {
                        val[j] = nums[i];
                        ++cnt[j];
                        break;
                    }
                }
            }
            if (cnt[k - 1] == 1) {
                for (int kk = 0; kk < k; ++kk) {
                    --cnt[kk];
                }
            }
        }
        vector<int> res;
        for (int i = 0; i < k; ++i) if (cnt[i] > 0) {
                int c = 0;
                for (auto n : nums) if (val[i] == n) ++c;
                if (c > nums.size() / k) res.push_back(val[i]);
        }
        return res;
    }
};