1. Two Sum

Given an array of integers, return indices of the two numbers such that they add up to a specific target.
You may assume that each input would have exactly one solution, and you may not use the same element twice.
Example:

Given nums = [2, 7, 11, 15], target = 9,
Because nums[0] + nums[1] = 2 + 7 = 9,
return [0, 1].

public int[] twoSum(int[] nums, int target) {
    int[] result = new int[2];
    Map<Integer, Integer> map = new HashMap<Integer, Integer>();
    for (int i = 0; i < nums.length; i++) {
// second find if containsKey ,get the value( which is index of nums[] )
        if (map.containsKey(target - nums[i])) {
            result[1] = i ;
            result[0] = map.get(target - nums[i]);
            return result;
        }
// first initialize the hashMap and put all (key ,value)
        map.put(nums[i], i );
    }
    return result;
}