_0001TwoSum.java

package com.heatwave.leetcode.problems;

import java.util.Arrays;
import java.util.HashMap;

/**
 * 1. Two Sum
 * Given an array of integers, return indices of the two numbers such that they add up to a specific target.
 * <p>
 * You may assume that each input would have exactly one solution, and you may not use the same element twice.
 * <p>
 * Example:
 * <p>
 * Given nums = [2, 7, 11, 15], target = 9,
 * <p>
 * Because nums[0] + nums[1] = 2 + 7 = 9,
 * return [0, 1].
 */
public class _0001TwoSum {
    static class Solution {
        public int[] twoSum(int[] nums, int target) {
            int[] res = {0, 0};
            for (int i = 0; i < nums.length; i++) {
                for (int j = i + 1; j < nums.length; j++) {
                    if (nums[i] + nums[j] == target) {
                        res[0] = i;
                        res[1] = j;
                        return res;
                    }
                }
            }
            return res;
        }
    }

    static class SolutionHash {
        public int[] twoSum(int[] nums, int target) {
            HashMap<Integer, Integer> hashMap = new HashMap<>();
            for (int i = 0; i < nums.length; i++) {
                hashMap.put(nums[i], i);
            }

            for (int i = 0; i < nums.length; i++) {
                int val = target - nums[i];
                Integer valIndex = hashMap.get(val);
                if (valIndex != null && valIndex != i) {
                    return new int[]{i, valIndex};
                }
            }
            throw new IllegalArgumentException("no answer");
        }
    }

    static class SolutionHashOnce {
        public int[] twoSum(int[] nums, int target) {
            HashMap<Integer, Integer> hashMap = new HashMap<>();
            for (int i = 0; i < nums.length; i++) {
                int val = target - nums[i];
                Integer valIndex = hashMap.get(val);
                if (valIndex != null && valIndex != i) {
                    return new int[]{valIndex, i};
                }

                hashMap.put(nums[i], i);
            }
            throw new IllegalArgumentException("no answer");
        }
    }

    public static void main(String[] args) {
        Solution solution = new Solution();
        int[] nums = {2, 7, 11, 15};
        int target = 13;
        int[] res = solution.twoSum(nums, target);
        System.out.println(Arrays.toString(res));
    }
}