1.two-sum.java

/*
 * @lc app=leetcode id=1 lang=java
 *
 * [1] Two Sum
 *
 * https://leetcode.com/problems/two-sum/description/
 *
 * algorithms
 * Easy (57.06%)
 * Likes:    67763
 * Dislikes: 2519
 * Total Accepted:    20.8M
 * Total Submissions: 36.4M
 * Testcase Example:  '[2,7,11,15]\n9'
 *
 * 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]
 * 
 * 
 * 
 * Constraints:
 * 
 * 
 * 2 <= nums.length <= 10^4
 * -10^9 <= nums[i] <= 10^9
 * -10^9 <= target <= 10^9
 * Only one valid answer exists.
 * 
 * 
 * 
 * Follow-up: Can you come up with an algorithm that is less than O(n^2) time
 * complexity?
 */

// @lc code=start
class Solution {
    public int[] twoSum(int[] nums, int target) {
        // edge cases
        // does the array contain -ves?
        
        // given array num = {2,7,11,15} and target 9 we want to retun the index 
        //of two elements that when added result to 9 which is our target

        // solutions
        // brute-force - initalize a pointer  i loop through the array from index 0 to num.length()-1
        //and pointer j to loop throguh from index i+1
        // if [i] + [j]+1 != 9 i++
        // else return [i],[j]

        //optimized
        //use hash map
        // instead of looking for the second number, we subtract the current element from the target 
        // to get the complement.
        
        // complement = target - [i];

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