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  1. import java.util.HashMap;
  2. import java.util.Map;
  3. import java.util.Set;
  4. import java.util.HashSet;
  5.  
  6. class Solution {
  7.     public int maxOperations(int[] nums) {
  8.         // Count occurrences of each number
  9.         Map<Integer, Integer> countMap = new HashMap<>();
  10.         for (int num : nums) {
  11.             countMap.put(num, countMap.getOrDefault(num, 0) + 1);
  12.         }
  13.  
  14.         // Generate all unique pair sums
  15.         Set<Integer> uniqueSums = new HashSet<>();
  16.         int n = nums.length;
  17.         for (int i = 0; i < n; i++) {
  18.             for (int j = i + 1; j < n; j++) {
  19.                 uniqueSums.add(nums[i] + nums[j]);
  20.             }
  21.         }
  22.  
  23.         int maxPairs = 0;
  24.  
  25.         // Count pairs for each unique sum using the precomputed counts
  26.         for (int k : uniqueSums) {
  27.             maxPairs = Math.max(maxPairs, countPairsForK(countMap, k));
  28.         }
  29.  
  30.         return maxPairs;
  31.     }
  32.  
  33.     private int countPairsForK(Map<Integer, Integer> countMap, int k) {
  34.         int pairCount = 0;
  35.  
  36.         for (Map.Entry<Integer, Integer> entry : countMap.entrySet()) {
  37.             int num = entry.getKey();
  38.             int partner = k - num;
  39.  
  40.             if (countMap.containsKey(partner)) {
  41.                 if (num == partner) {
  42.                     // If both numbers are the same, use combination formula nC2
  43.                     pairCount += entry.getValue() / 2;
  44.                 } else if (num < partner) { // To avoid double counting
  45.                     // Count the pairs using the minimum of the counts of the two numbers
  46.                     pairCount += Math.min(entry.getValue(), countMap.get(partner));
  47.                 }
  48.             }
  49.         }
  50.  
  51.         return pairCount;
  52.     }
  53.  
  54.     public static void main(String[] args) {
  55.         Solution solution = new Solution();
  56.         // Example input
  57.         int[] nums = {1, 2, 3, 4, 3, 2, 1};
  58.         int result = solution.maxOperations(nums);
  59.         System.out.println(result); // Output the maximum number of pairs
  60.     }
  61. }
  62.  
Success #stdin #stdout 0.09s 54576KB
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https://ideone.com/PUliOo
language:
Java (HotSpot 12)
created:
2 days ago
visibility:
public

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