[Algo] 182. 2 Sum All Pair II

Find all pairs of elements in a given array that sum to the pair the given target number. Return all the distinct pairs of values.

Assumptions

• The given array is not null and has length of at least 2
• The order of the values in the pair does not matter

Examples

• A = {2, 1, 3, 2, 4, 3, 4, 2}, target = 6, return [[2, 4], [3, 3]]

Time: O(N)

public class Solution {
  public List<List<Integer>> allPairs(int[] array, int target) {
    // Write your solution here
        List<List<Integer>> res = new ArrayList<>();
      Map<Integer, Integer> map = new HashMap<>();
      for (int num : array) {
        Integer freq = map.get(num);
        int other = target - num;
        if (other != num && map.containsKey(other) && freq == null) {
          res.add(Arrays.asList(other, num));
        } else if (num + num == target && freq != null && freq == 1) {
          res.add(Arrays.asList(num, num));
        }
        if (freq == null) {
          map.put(num, 1);
        } else {
          map.put(num, freq + 1);
        }
      }
      return res;
  }
}

Time: O(NlogN)

public class Solution {
    /**
     * @param nums: an array of integer
     * @param target: An integer
     * @return: An integer
     */
    public int twoSum6(int[] array, int target) {
        // write your code here
            if (array == null || array.length <= 1) {
                return 0;
            }
            Arrays.sort(array);
            int res = 0;
            int left = 0, right = array.length - 1;
            while (left < right) {
                int tmpSum = array[left] + array[right];
                if (tmpSum == target) {
                    res += 1;
                    left += 1;
                    while (left <= right && array[left] == array[left - 1]) {
                        left += 1;
                    }
                } else if (tmpSum < target) {
                    left += 1;
                } else {
                    right -= 1;
                }
            }
            return res;

    }
}