Two Sum: find two number from array of integers and thats sum equal to target integer

258 viewsProblem Solving

Two Sum: find two number from array of integers and thats sum equal to target integer

Given an array of integers nums and an integer target, return indices of the two numbers such that they add up to target.

You may assume that each input would have exactly one solution, and you may not use the same element twice.

You can return the answer in any order.

Example 1:

Input: nums = [2,7,11,15], target = 9
Output: [0,1]
Explanation: Because nums[0] + nums[1] == 9, we return [0, 1].

Example 2:

Input: nums = [3,2,4], target = 6
Output: [1,2]

Example 3:

Input: nums = [3,3], target = 6
Output: [0,1]

Farjanul Changed status to publish July 27, 2022
0

O(n2) runtime, O(1) space – Brute force:

The brute force approach is simple. Loop through each element x and find if there is another value that equals to target – x. As finding another value requires looping through the rest of array, its runtime complexity is O(n2).

O(n) runtime, O(n) space – Hash table:
We could reduce the runtime complexity of looking up a value to O(1) using a hash map

that maps a value to its index.

Solution in Java

public class Solution {
    // example in leetcode book
    public int[] twoSum(int[] nums, int target) {
        Map<Integer, Integer> map = new HashMap<>();
        for (int i = 0; i < nums.length; i++) {
            int x = nums[i];
            if (map.containsKey(target - x)) {
                return new int[]{map.get(target - x), i};
            }
            map.put(x, i);
        }
        throw new IllegalArgumentException("No two sum solution");
    }
}

Solution in Python

class Solution(object):
    def twoSum(self, nums, target):
        # two point
        nums_index = [(v, index) for index, v in enumerate(nums)]
        nums_index.sort()
        begin, end = 0, len(nums) - 1
        while begin < end:
            curr = nums_index[begin][0] + nums_index[end][0]
            if curr == target:
                return [nums_index[begin][1], nums_index[end][1]]
            elif curr < target:
                begin += 1
            else:
                end -= 1
 if __name__ == '__main__':
    # begin
    s = Solution()
    print s.twoSum([3, 2, 4], 6)

Farjanul Changed status to publish July 27, 2022
0
You are viewing 1 out of 1 answers, click here to view all answers.