LeetCode Majority Element I && II

原题链接在这里：https://leetcode.com/problems/majority-element/

题目：

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.

题解：

Method 1：最容易想到的就是用HashMap 计数，数值大于n/2(注意不是大于等于而是大于)，就是返回值。Time Complexity: O(n). Space: O(n).

Method 2: 用sort, 返回sort后array的中值即可. Time Complexity: O(n*logn). Space: O(1).

Method 3: 维护个最常出现值，遇到相同count++, 遇到不同count--, count为0时直接更改最常出现值为nums[i]. Time Complexity: O(n). Space: O(1).

AC Java:

 public class Solution {
     public int majorityElement(int[] nums) {
         /*
         //Method 1, HashMap
         HashMap<Integer, Integer> map = new HashMap<>();
         for(int i = 0;i<nums.length; i++){
             if(!map.containsKey(nums[i])){
                 map.put(nums[i],1);
             }else{
                 map.put(nums[i],map.get(nums[i])+1);
             }
         }

         Iterator<Integer> it = map.keySet().iterator(); //Iterate HashMap
         while(it.hasNext()){
             int keyVal = it.next();
             //There is an error here: Pay attentation, it is ">", but not ">="
             //If we have three variables [3,2,3], ">=" will also return 2, 1>=3/2
             if(map.get(keyVal) > nums.length/2){
                 return keyVal;
             }
         }

         return Integer.MIN_VALUE;
         */

         /*Method 2, shortcut
         Arrays.sort(nums);
         return nums[nums.length/2];
         */

         //Method 3
         if(nums == null || nums.length == 0)
             return Integer.MAX_VALUE;
         int res = nums[0];
         int count = 1;
         for(int i = 1; i< nums.length; i++){
             if(res == nums[i]){
                 count++;
             }else if(count == 0){
                 res = nums[i];
                 count = 1;
             }else{
                 count--;
             }
         }
         return res;

     }
 }

跟上Majority Element II.

类似Check If a Number Is Majority Element in a Sorted Array.