15. 3Sum

LeetCode
,

Two Pointers

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class Solution {
    public List<List<Integer>> threeSum(int[] nums) {
        // sort the input array
        Arrays.sort(nums);
        // result 
        List<List<Integer>> result = new ArrayList<>();
        int n = nums.length;

        for (int i = 0; i < n - 2; i++) {
            if (i > 0 && nums[i] == nums[i - 1]) continue;
            int lP = i + 1;
            int rP = n - 1;

            while (lP < rP) {
                int sum = nums[i] + nums[lP] + nums[rP];
                if (sum == 0){
                    result.add(Arrays.asList(nums[i], nums[lP], nums[rP]));
                    while (lP < rP && nums[lP] == nums[lP+1]) lP++;
                    while (lP < rP && nums[rP] == nums[rP-1]) rP--;
                    lP++;
                    rP--;
                } else if (sum < 0){
                    lP++;
                } else {
                    rP--;
                }
            }

        }

        return result;
    }
}

Remarks:

  1. Sorting an array in Java: Arrays.sort(nums);
  2. Use while loops to skip the duplicated numbers
  3. Use to pointers to get the smallest sum