LeetCode 1. 两数之和

1. 两数之和

Solution

思路1：直接双重for循环，枚举每个数字，找到后返回结果

class Solution {
    public int[] twoSum(int[] nums, int target) {
        int len = nums.length;
        for (int i = 0; i < len; i++) {
            for (int j = i + 1; j < len; j++) {
                if (nums[i] + nums[j] == target) {
                    return new int[]{i, j};
                }
            }
        }
        return null;
    }
}

思路2：利用哈希表存当前的值，然后判断需要的另一个数，是否在哈希表内即可

//这种写法要判断 4 +4 = 8的情况
class Solution {
    public int[] twoSum(int[] nums, int target) {
        Map<Integer, Integer> map = new HashMap<>();
        int len = nums.length;
        for (int i = 0; i < len; i++) {
            map.put(nums[i], i);
        }
        for (int i = 0; i < len; i++) {
            if (map.containsKey(target - nums[i]) && (map.get(target - nums[i]) != i)) {
                return new int[]{map.get(target - nums[i]), i};
            }
        }
        return null;
    }
}

// 这种不用
    class Solution {
        public int[] twoSum(int[] nums, int target) {
            Map<Integer, Integer> map = new HashMap<>();
            int len = nums.length;
            for (int i = 0; i < len; i++) {
                if (map.containsKey(target - nums[i])) {
                    return new int[]{map.get(target - nums[i]), i};
                } else {
                    map.put(nums[i], i);
                }
            }
            return null;
        }
    }