229. Majority Element II (M)

Given an integer array of size n, find all elements that appear more than ⌊ n/3 ⌋ times.

Follow-up: Could you solve the problem in linear time and in O(1) space?

Example 1:

Input: nums = [3,2,3]
Output: [3]
Example 2:

Input: nums = [1]
Output: [1]
Example 3:

Input: nums = [1,2]
Output: [1,2]

Constraints:

1 <= nums.length <= 5 * 104
-109 <= nums[i] <= 109

---

我的答案：时间、空间复杂度O(n)

class Solution {
public:
    vector<int> majorityElement(vector<int>& nums) {
        int length = nums.size();
        
        unordered_map<int, int> count;
        unordered_set<int> screen_out;
        
        for (const auto& n:nums) {
            if (screen_out.find(n) == screen_out.end()) {
                if (count.find(n) == count.end()) {
                    count[n] = 1;
                }
                else {
                    ++count[n];
                }
                if (count[n] > length/3) {
                    screen_out.insert(n);
                }
            }
        }
        
        vector<int> ans(screen_out.begin(), screen_out.end());
        return ans;
    }
};

Runtime: 40 ms, faster than 26.49% of C++ online submissions for Majority Element II.
Memory Usage: 16.5 MB, less than 7.86% of C++ online submissions for Majority Element II.

有点慢

看答案：
https://zxi.mytechroad.com/blog/algorithms/array/leetcode-229-majority-element-ii/

Solution: Boyer–Moore Voting Algorithm
Time complexity: O(n)
Space complexity: O(1)

class Solution {
public:
    vector<int> majorityElement(vector<int>& nums) {
        int length = nums.size();
        
        unordered_map<int, int> count;
        vector<int> ans;
        // ans.reserve(2);
        
        for (const auto& n:nums) {
            if (find(ans.begin(), ans.end(), n) == ans.end()) {
                if (count.find(n) == count.end()) {
                    count[n] = 1;
                }
                else {
                    ++count[n];
                }
                if (count[n] > length/3) {
                    ans.push_back(n);
                    if (ans.size() == 2) break;
                }
            }
        }
        
        return ans;
    }
};

Runtime: 32 ms, faster than 51.59% of C++ online submissions for Majority Element II.
Memory Usage: 16.3 MB, less than 7.86% of C++ online submissions for Majority Element II.

看了答案之后发现，最多有2个元素输出
做了点优化

---

答案Boyer-Moore算法：

// Author: Huahua
class Solution {
public:
  vector<int> majorityElement(vector<int>& nums) {
    int n1 = 0;
    int c1 = 0;
    int n2 = 1;
    int c2 = 0;
    for (int num : nums) {
      if (num == n1) {
        ++c1;
      } else if (num == n2) {
        ++c2;
      } else if (c1 == 0) {
        n1 = num;
        c1 = 1;
      } else if (c2 == 0) {
        n2 = num;
        c2 = 1;
      } else {
        --c1;
        --c2;
      }
    }
    
    c1 = c2 = 0;
    for (int num : nums) {
      if (num == n1) ++c1;
      else if (num == n2) ++c2;
    }
    
    const int c = nums.size() / 3;
    vector<int> ans;
    if (c1 > c) ans.push_back(n1);
    if (c2 > c) ans.push_back(n2);
    return ans;
  }
};

Runtime: 28 ms, faster than 76.28% of C++ online submissions for Majority Element II.
Memory Usage: 16.1 MB, less than 23.62% of C++ online submissions for Majority Element II.

问题：
为什么要重新计数？
https://www.youtube.com/watch?v=FGwCv6JAZQ8

实验了一下，果然出错了！主要是count可能会降到0，这时候有可能整个需要，有可能不需要

然后上面的代码其实有点tricky，n1和n2需要初始化，但是要初始化成两个不一样的数字，类似前面都搜索过一段了，但是刚好两个counter都清零的结果。否则如果n1=n2=0，然后输入都是0，那么第二个for循环的第二个if就得是else if，这样也行。

最后是，第一个for loop里面if和else if的顺序，必须把n1和n2的判断放到最前面，否则

        for (const auto& n:nums){
            if (c1 == 0) {
                n1 = n;
                ++c1;
            }
            else if (c2 == 0) {
                n2 = n;
                ++c2;
            }
            else if (n == n1) ++c1;
            else if (n == n2) ++c2;
            else {
                --c1;
                --c2;
            }
        }

比如连续两个1输入，[1,1,...]，那么第二个1就会把c2也占了，而且本身的计数也会少加了1

cout << n << ", " << n1 << ", " << c1 << ", " << n2 << ", " << c2 << endl;
错误结果：
1, 1, 1, 2147483647, 0
1, 1, 1, 1, 1
2, 1, 0, 1, 0 <~ “那么第二个1就会把c2也占了，而且本身的计数也会少加了1”
3, 3, 1, 1, 0
4, 3, 1, 4, 1
1, 3, 0, 4, 0
1, 1, 1, 4, 0
5, 1, 1, 5, 1
6, 1, 0, 5, 0
7, 7, 1, 5, 0
1, 7, 1, 1, 1
1, 7, 1, 1, 2
8, 7, 0, 1, 1
9, 9, 1, 1, 1
10, 9, 0, 1, 0
1, 1, 1, 1, 0
11, 1, 1, 11, 1
12, 1, 0, 11, 0
13, 13, 1, 11, 0
14, 13, 1, 14, 1
正确结果
1, 1, 1, 2147483647, 0
1, 1, 2, 2147483647, 0
2, 1, 2, 2, 1
3, 1, 1, 2, 0
4, 1, 1, 4, 1
1, 1, 2, 4, 1
1, 1, 3, 4, 1
5, 1, 2, 4, 0
6, 1, 2, 6, 1
7, 1, 1, 6, 0
1, 1, 2, 6, 0
1, 1, 3, 6, 0
8, 1, 3, 8, 1
9, 1, 2, 8, 0
10, 1, 2, 10, 1
1, 1, 3, 10, 1
11, 1, 2, 10, 0
12, 1, 2, 12, 1
13, 1, 1, 12, 0
14, 1, 1, 14, 1

---

回顾169. Majority Element

Boyer moore的写法是：

// https://zxi.mytechroad.com/blog/divide-and-conquer/leetcode-169-majority-element/
class Solution {
public:
    int majorityElement(vector<int>& nums) {
        int majority = nums.front();
        int count = 0;
        
        for (const int num : nums) {
            if (num == majority) ++count;
            else if (--count == 0) {
                count = 1;
                majority = num;
            }
        }
        
        return majority;
    }
};

这里的count判断和num不需要注意前后顺序，因为只有一个count

我自己的写的版本，过了测试

class Solution {
public:
    int majorityElement(vector<int>& nums) {
        int majority = nums.front();
        int count = 0;
        
        for (const int num : nums) {
            if (count == 0) {
                ++count;
                majority = num;
            }
            else if (num == majority) ++count;
            else --count;
        }
        
        return majority;
    }
};