Friedrich Ewald My Personal Website


  • Leetcode: Move zeros

    The task is as follows: Given an integer array nums, move all 0’s to the end of it while maintaining the relative order of the non-zero elements. Note that you must do this in-place without making a copy of the array. The last note is actually the important part. Without this, it would be easy to just use array splitting in Python and add a zero. This would look similar to this:

    if nums[pos] == 0:
      nums = nums[:pos] + nums[pos+1:] + [0]
    This obviously overrides nums and the change is no longer in place. The next best solution that I found was to delete the items at position I from the list and then add a zero to the end. It is important to note, that the index needs to be decremented by one in case a 0 is found to account for double 0. The number of steps needs to always be increased.

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  • Leetcode: Reverse string

    Reverse a string and only use one extra variable. The solution is straightforward with two pointers, p1 and p2. THe idea here is to start from both ends and increment p1 and decrement p2 until both of them meet in the middle. To switch the values of variables without assigning a temporary variable, the notation of a, b = b, a can be used. To convert a string into a list of characters, surroung the string with [*s].

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  • Leetcode: Majority number

    Given an array nums of size n, return the majority element. The majority element is the element that appears more than ⌊n / 2⌋ times. You may assume that the majority element always exists in the array. Unfortunately, it is not clear from the description whether there are only two different numbers or multiple ones. Some of the solutions seem to suggest that there are only two different ones. The solution that I will show has a time complexity of O(n) and a space complexity of O(n) in the worst case, that is, if all the numbers appear exactly once.

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  • Leetcode: Single number

    Given a list of integers [1,1,2], there is only one number which is not repeated. The numbers can be in any order. The minimum length of the array is 1. Numbers can be negative or positive. The algorithm should have a time complexity of O(n) and a space complexity should be constant O(1). Given this information, we can use the properties of a set or a map.

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  • Leetcode: First unique character in string

    Given a string s, find the first non-repeating character in it and return its index. If it does not exist, return -1. An example is the word leetcode where the l only appears once, therefore returning 0 as result. The obvious, but very slow, solution is to iterate over the string and then for ever character, check the whole string if the character appears again. This has a time complexity of O(n^2). A better solution is to write all characters in a map and count the number of occurrences. This way, we can iterate again and simply look at the counter in the map. This yields a time complexity of O(2n).

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