leetcode 面試題 17.10. 主要元素
阿新 • • 發佈:2021-07-10
陣列中佔比超過一半的元素稱之為主要元素。給你一個 整數 陣列,找出其中的主要元素。若沒有,返回 -1
。請設計時間複雜度為 O(N)
、空間複雜度為 O(1)
的解決方案。
示例 1:
輸入:[1,2,5,9,5,9,5,5,5] 輸出:5
示例 2:
輸入:[3,2] 輸出:-1
示例 3:
輸入:[2,2,1,1,1,2,2] 輸出:2
方法一:利用雜湊統計每個元素出現的次數
/** * 方法一:雜湊 * * @param nums * @return */ public static int majorityElement01(int[] nums) { if (nums == null || nums.length == 0) return 0; Map<Integer, Integer> integerMap = new HashMap<>(); for (int i = 0; i < nums.length; i++) { integerMap.put(nums[i], integerMap.getOrDefault(nums[i], 0) + 1); if (integerMap.get(nums[i]) > nums.length / 2) return nums[i]; } return -1; }
方法二:利用摩爾投票
當一個數的重複次數超過陣列長度的一半,每次將兩個不相同的數刪除,最終剩下的就是要找的數
/** * 摩爾投票法 * * @param nums * @return */ public static int majorityElement(int[] nums) { if (nums == null || nums.length == 0) return 0; int target = nums[0]; int count = 1; for (int i = 1; i < nums.length; i++) { if (nums[i] == target) count++; else { count--; if (count <= 0) { target = nums[i]; count = 1; } } } count = 0; for (int num : nums) { if (num == target) count++; } return count > nums.length / 2 ? target : -1; }
測試用例
public static void main(String[] args) { int[] nums = new int[]{1, 2, 5, 9, 5, 9, 5, 5, 5}; int majorityElement = MajorityElement.majorityElement(nums); System.out.println("MajorityElement demo01 result : " + majorityElement); nums = new int[]{3, 2}; majorityElement = MajorityElement.majorityElement(nums); System.out.println("MajorityElement demo02 result : " + majorityElement); nums = new int[]{3, 3, 2}; majorityElement = MajorityElement.majorityElement(nums); System.out.println("MajorityElement demo03 result : " + majorityElement); nums = new int[]{3, 2, 2}; majorityElement = MajorityElement.majorityElement(nums); System.out.println("MajorityElement demo04 result : " + majorityElement); nums = new int[]{3, 2, 3}; majorityElement = MajorityElement.majorityElement(nums); System.out.println("MajorityElement demo05 result : " + majorityElement); nums = new int[]{2, 2, 1, 1, 1, 2, 2}; majorityElement = MajorityElement.majorityElement(nums); System.out.println("MajorityElement demo06 result : " + majorityElement); }