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MajorityElement.cpp
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MajorityElement.cpp
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#include <vector>
#include <unordered_map>
using namespace std;
class Solution {
public:
/*Sol 0
* Moore Voting Algorithm
* The key is majority element appears more than 1/2 times of all.
* Space complexity O(1), time complexity O(n)
*/
int majorityElement(vector<int>& nums) {
int majority = nums[0],count = 1;
for(int i = 1;i < nums.size();++i){
if(count == 0){
majority = nums[i];
++count;
}
else if(majority == nums[i]) ++count;
else --count;
}
return majority;
}
/* Sol 1
* Hash
* Time complexity O(n),space complexity O(n)
*/
int majorityElement1(vector<int>& nums){
unordered_map<int,int> map;
for(auto num : nums){
if(++map[num] > nums.size() / 2)
return num;
}
return nums[0];//control reaches non-void function, treat this as error
}
/* Sol 2
* Random Algorithm
* It's also fast to get the majority element.
*/
int majorityElement2(vector<int>& nums){
int len = nums.size();
srand(time(NULL));
while(true){
int seed = nums[rand() % len];
int count = 0;
for(auto num : nums){
if(num == seed) ++count;
}
if(count > len / 2) return seed;
}
}
/* Sol 3
* Sort
* Easy to implement.
* time complexity O(nlog(n)), space complexity O(1)
*/
};