leetcode1.两数之和

题目

时间复杂度O(N^2)

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};
                }
            }
        }
        throw new IllegalArgumentException("No two sum solution");
    }
}

           

时间复杂度O(N)

空间换时间

class Solution {
    public int[] twoSum(int[] nums, int target) {
        HashMap<Integer,Integer> map = new HashMap<>();
        for(int i = 0;i < nums.length;i++){
            if(map.containsKey(target-nums[i])){
                return new int[]{i,map.get(target-nums[i])};
            }
            map.put(nums[i],i);
        }
        throw new IllegalArgumentException("No two sum solution");
    }
}