LeetCode -1.两数之和

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LeetCode- 1. 两数之和

题目

给定一个整数数组 nums 和一个目标值 target，请你在该数组中找出和为目标值的那 两个 整数，并返回·他们的数组下标。
你可以假设每种输入只会对应一个答案。但是，你不能重复利用这个数组中同样的元素。

思考

  • 正向思维：
    • 要想得到target，就需要事先确定两个值，假设为num[i]、num[j]
    • 由于题意说每种输入只会对应一个答案，则说明这两个值处于整数数组的不同位置。
    • 所以使用两层循环依次遍历，即可确定在哪两个位置的整数之和等于target.
  • 逆向思维:
    • target是已知的，减去一个整数num[i]，就能求出另一个整数,假设为tmp.
    • 如果tmp在数组中，就能找到答案.

题解 1

class Solution {
    public int[] twoSum(int[] nums, int target) {
        int[] res = new int[2];
        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;
                    break;
                }
            }
        }
        return res;
    }
}

题解 2

class Solution {
    public int[] twoSum(int[] nums, int target) {
        Map<Integer, Integer> map = new HashMap<Integer, Integer>();
        int[] res = new int[2];
        for(int i = 0; i < nums.length; i ++) {
            map.put(nums[i], i);
        }
        for(int i = 0; i < nums.length; i ++) {
            int tmp = target - nums[i]; 
            if(map.containsKey(tmp) && map.get(tmp) != i) {
                res[0] = i;
                res[1] = map.get(tmp);
                break;
            }
        }
        return res;
    }
}
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posted @ 2020-02-03 18:10  tumint  阅读(89)  评论(0)    收藏  举报