Solution.java

//Given an array of integers, return indices of the two numbers such that they a
//dd up to a specific target. 
//
// You may assume that each input would have exactly one solution, and you may n
//ot 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].
// 
// Related Topics Array Hash Table


import java.util.HashMap;

//leetcode submit region begin(Prohibit modification and deletion)
class Solution {
    public int[] twoSum(int[] nums, int target) {
        // BFS, O(n^2)
//        return bfs(nums, target);

        // HashMap, O(n)
        return hashMap(nums, target);
    }

    private int[] hashMap(int[] nums, int target) {
        HashMap<Integer, Integer> map = new HashMap<>();
        // map<num, pos>
        for (int i = 0; i < nums.length; i++) {
            int n = target - nums[i];
            if (map.containsKey(n)) {
                return new int[]{map.get(n), i};
            }
            map.put(nums[i], i);
        }
        return new int[]{2};
    }

    private int[] bfs(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[2];
    }
}
//leetcode submit region end(Prohibit modification and deletion)