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 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");
    }