Friedrich Ewald My Personal Website


  • 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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  • Leetcode: Symmetric tree

    The task is to determine if a tree is symmetric, meaning mirrored along the root node. The usual tree structure is given in form of a TreeNode.

    class TreeNode:
      def __init__(self, val=0, left=None, right=None):
        self.val = val
        self.left = left
        self.right = right
    A symmetric tree looks like this:
    Note that the 3 and 4 are flipped in the left vs. right branch.
           /   \
          2     2
         / \   / \
        3   4 4   3
    An asymmetric tree looks like this:
    Note that the 3 would need to be on the other side of 
    either branch to make this tree symmetric.
           /   \
          2     2
         /     /
        3     3
    The logic itself is quite straightforward look at two nodes and compare the values. If their respective values are the same look at both of their children and compare them. Repeat this process until every node is visited. There are some special cases here that need to be handled. The left child can be None while the right child is not and vice versa. If both children are None, return True and do not proceed because this means we reached a leaf node.

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