1. Sum of two numbers

1. Violence solution

/** *  describe   Violence solution  O(n^2) */
class Solution {
    
    public int[] twoSum(int[] nums, int target) {
    
        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 new int[]{
    };
    }

    public static void main(String[] args) {
    
        Solution solution = new Solution();
        int[] ints = solution.twoSum(new int[]{
    2, 7, 11, 15}, 9);
        System.out.println(ints[0] + " " + ints[1]);
    }
}

2. Hashtable

/** *  Project name ： leetcode *  Package name : com.kcl.L1 *  file name  Solution *  The creator  *  Creation time : 2022/7/10 10:49 AM *  describe  hash O(n) */
class Solution2 {
    
    public int[] twoSum(int[] nums, int target) {
    
        Map<Integer,Integer> map = new HashMap<>();
        int len = nums.length;
        for(int i = 0; i < len; i++){
    
            // Look forward to 
            if(map.get(target-nums[i]) != null){
    
               return new int[]{
    i,map.get(target-nums[i])};
            }else{
    
                map.put(nums[i],i);
            }
        }
        return new int[]{
    };
    }

    public static void main(String[] args) {
    
        Solution2 solution = new Solution2();
        int[] ints = solution.twoSum(new int[]{
    2, 7, 11, 15}, 9);
        System.out.println(ints[0] + " " + ints[1]);
    }
}