描述

https://leetcode-cn.com/problems/two-sum/

我的解

class Solution {
    public int[] twoSum(int[] nums, int target) {
        Map<Integer, List<Integer>> map = new HashMap<>();
        for (int i = 0; i < nums.length; i++) {
            int num = nums[i];
            if (map.containsKey(target - num)) {
                int[] ans = new int[2];
                List<Integer> list = map.get(target - num);
                ans[0] = i;
                for (int j : list) {
                    if (j != i) {
                        ans[1] = j;
                        return ans;
                    }
                }
            }
            map.putIfAbsent(num, new ArrayList<>());
            map.get(num).add(i);
        }
        return new int[2];
    }
}

次优解

class Solution {
    public int[] twoSum(int[] nums, int target) {
        Map<Integer, Integer> hashtable = new HashMap<Integer, Integer>();
        for (int i = 0; i < nums.length; ++i) {
            if (hashtable.containsKey(target - nums[i])) {
                return new int[]{hashtable.get(target - nums[i]), i};
            }
            hashtable.put(nums[i], i);
        }
        return new int[0];
    }
}

最优解

class Solution {
    public int[] twoSum(int[] nums, int target) {
        int len = nums.length;
        int hlen = len/2;
        int toFind1;
        int toFind2;
        int bu;
        for (int i = 0; i < hlen; i++) {
            bu = len-i-1;
            toFind1 = target - nums[i];
            toFind2 = target - nums[bu];
            for (int j = i + 1; j < len; j++) {
                if (nums[j]==toFind1) {
                    return new int[]{i,j};
                }
                if(nums[j]==toFind2){
                    return new int[]{j,bu};
                }
            }
        }
        return null;
    }
}