【Tour of LeetCode】Q1——Two Sum

2024年6月9日 51次阅读

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

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