Coding tests are a process that modern programmers inevitably go through. Among them, understanding and utilizing time complexity is very important. In this article, we will explain in detail how to solve algorithm problems using Java, along with how to analyze time complexity.<\/p>\n
Given an integer array This problem can be solved in two ways. The first method is using a double loop, and the second is using a hashmap. We will analyze the time complexity of each method and implement the hashmap method first.<\/p>\n This method checks all pairs in the array to see if their sum equals In the double loop, each element is checked against all other elements. Therefore, the time complexity is To reduce time complexity, we can use a hashmap to solve this problem. This method stores the indices of each element and checks if the value of The time complexity of this method is This problem highlighted the importance of time complexity. While the double loop approach is simple, it can lead to performance degradation, so it is necessary to use optimized data structures like hashmaps in practice.<\/p>\nnums<\/code> and an integer target<\/code>, the problem is to find a pair of indices such that the two numbers in the array sum to target<\/code>. If there is none, return -1, -1<\/code>.<\/p>\nProblem Requirements<\/h3>\n
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Example<\/h3>\n
\nInput: nums = [2, 7, 11, 15], target = 9\nOutput: [0, 1]\nExplanation: nums[0] + nums[1] = 2 + 7 = 9, so return [0, 1].\n<\/code><\/pre>\nSolution Approach<\/h2>\n
Method 1: Double Loop<\/h3>\n
target<\/code>. The code implementation is as follows:<\/p>\npublic int[] twoSum(int[] nums, int target) {\n for (int i = 0; i < nums.length; i++) {\n for (int j = i + 1; j < nums.length; j++) {\n if (nums[i] + nums[j] == target) {\n return new int[]{i, j};\n }\n }\n }\n return new int[]{-1, -1};\n}<\/code><\/pre>\nTime Complexity Analysis<\/h3>\n
O(n^2)<\/code>. This can be inefficient in the worst case.<\/p>\nMethod 2: Using Hashmap<\/h3>\n
target - nums[i]<\/code> exists as a key, returning the index if it does.<\/p>\nimport java.util.HashMap;\n\npublic int[] twoSum(int[] nums, int target) {\n HashMapTime Complexity Analysis<\/h3>\n
O(n)<\/code>. Since each element is checked only once, it is efficient. The insertion and retrieval from the hashmap are performed in constant time, making this approach superior.<\/p>\nComplexity Analysis and Conclusion<\/h2>\n
Additional Tips<\/h3>\n