【Tour of LeetCode】Q1——Two Sum-优快云博客

本文探讨了在给定整数数组中寻找两个数使其和等于特定目标值的问题，通过三次提交逐步优化算法，从原始的暴力求解到时间优化后的解决方案，最后提出进一步的算法改进思路。

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Question

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 the same element twice.

Example:

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

Solution 1 : Brute Force

■ 1st Submission

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

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

Finished
Runtime: 0 ms
Your input
[2,7,11,15] 9
Output
[1,0]
Expected
[0,1]

Oops, [1] shows that I am not fully understanding the procedure of the "for" loop

■ 2nd Submission

Brute Force with correct "for" loop

class Solution {
    public int[] twoSum(int[] nums, int target) {
        for (int i = 0; i < nums.length; i++) {
            for (int j = 0; j < nums.length; j++) {    // [2]
                if (j == i) continue;
                if (nums[i] + nums[j] == target) return new int[]{i, j};
            }
        }

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

Status
Accepted
Runtime
75 ms
Memory
27.7 MB

[2] makes a "drawback" that causes unnecessary calculation
Time Complexity : O(n²), Space Complexity : O(1)

■ 3rd Submission

Time-optimized Brute Force

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

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

Status
Accepted
Runtime
35 ms
Memory
27.6 MB

Is there any other better algorithm?

Solution 2 : ?

See you later