3Sum

Given an array nums of n integers, are there elements a, b, c in nums such that a + b + c = 0? Find all unique triplets in the array which gives the sum of zero.

Solution1

Brute Force

public int[] twoSum(int[] nums, int target) {
    if (nums.length == 0) {
        return null;
    }

    for (int i = 0; i < nums.length; i++) {
        for (int j = i + 1; j < nums.length; j++) {
            if (nums[i] + nums[j] == target) {
                return new int[]{i, j};
            }
        }
    }
    return null;
}

Solution2

HashMap

public int[] twoSum(int[] nums, int target) {
    if (nums.length == 0) {
        return null;
    }

    Map<Integer, Integer> map = new HashMap<>();
    for (int i = 0; i < nums.length; i++) {
        int current = nums[i];
        int expect = target - current;
        if (map.containsKey(expect)) {
            return new int[]{i, map.get(expect)};
        }
        map.put(current, i);
    }
    return null;
}