leetcode刷题笔记-two sum

问题描述：

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

给定一个数组int[] nums，给定一个数target，要求返回数组中相加等于target的两个数的下标组成的数组：

解题思路：
用两层循环查找，时间复杂度O(n2)

    public int[] twoSum(int[] nums, int target) {        int[] a = new int[2];        boolean flag = false;        for (int i = 0; i < nums.length - 1; i++) {            for (int j = i + 1; j < nums.length; j++) {                if (nums[i] + nums[j] == target) {                    a[0] = i;                    a[1] = j;                    break;                }            }            if (flag)                break;        }        return a;    }

改进的算法，使用HashMap时间复杂度为O(n)

    public int[] twoSum(int[] nums, int target) {        Map<Integer, Integer> map = new HashMap<>();        for (int i = 0; i < nums.length; i++) {            int value = target - nums[i];            if (map.containsKey(value)) {                return new int[] { map.get(value), i };            }            map.put(nums[i], i);        }        throw new IllegalArgumentException("No such solution");    }