LeetCode 1.Two Sum

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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, and you may not use thesame element twice.

Example:

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

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方法一：

【双重for循环暴力破解】

    时间复杂度O(n^2);

方法二：

【空间换时间】

    时间复杂度O(n);

    public int[] twoSum(int[] nums, int target) {        HashMap<Integer, Integer> m = new HashMap<Integer, Integer>();        int[] res = new int[2];        for(int i = 0; i < nums.length; i++){            m.put(nums[i],i);        }        for (int i = 0; i < nums.length; i++) {            if (m.containsKey(target - nums[i]) && m.get(target - nums[i]) != i) {                res[0] = i;                res[1] = m.get(target - nums[i]);                break;            }        }        return res;    }

方法三：

对方法二的优化

    public int[] twoSum(int[] nums, int target) {        HashMap<Integer, Integer> m = new HashMap<Integer, Integer>();        int[] res = new int[2];        for (int i = 0; i < nums.length; i++) {            if (m.containsKey(target - nums[i])) {                res[0] = m.get(target - nums[i]);                res[1] = i;                break;            }            m.put(nums[i],i);        }        return res;    }