# All posts in Algorithms

Problem statement There are stairs, a person standing at the bottom wants to reach the top. The person can climb either stair or stairs at a time. Count the number of ways, the person can reach the top. Solution We can solve this problem recursively. If the person were to . . . Read more

Problem statement Given a linked list, determine if it has a cycle in it. Solution For the solution we are going to use a technique known as the two-pointer technique. That is we will involve two pointers: one slow-runner and the other fast-runner. The slow-runner will move forward 1 list . . . Read more

Problem statement Divide two integers without using multiplication, division and mod operator. If it is overflow, return MAX_INT. Solution Obviously the naive approach to this problem would be to subtract the divisor from the dividend until the dividend becomes less than the divisor, while keeping track of how many times . . . Read more

Problem statement Given a list, nums, of distinct numbers, return all possible unique permutations. Sample input [1,2,1] Sample output [[1, 2, 1], [2, 1, 1], [1, 1, 2]] Solution In another post we showed how this problem can be solved iteratively. We can use the same technique but add a . . . Read more

Problem statement Given a list, nums, of distinct numbers, return all possible permutations. Sample input [1,2,3] Sample output [ [1,2,3], [1,3,2], [2,1,3], [2,3,1], [3,1,2], [3,2,1] ] Solution In another post we showed how this problem can be solved using a recursive technique by swapping. We stumbled upon a nice iterative . . . Read more

Problem statement Given a list, nums, of distinct numbers, return all possible permutations. Sample input [1,2,3] Sample output [ [1,2,3], [1,3,2], [2,1,3], [2,3,1], [3,1,2], [3,2,1] ] Solution We are using a technique called permutations by swapping. Full code class Solution(object): def swap(self, nums, i, j): tmp = nums[i] nums[i] = . . . Read more

Problem statement You are given an 2D matrix / array representing an image. Rotate matrix by 90 degrees (clockwise) in-place. Solution We will work through the matrix layers. The layers of a matrix are illustrated below in different colours: There are a total of layers. We then iterate of the . . . Read more

Problem statement A time series is a series of data points indexed in time order. They are commonly used in the financial world, especially in stock markets. In this challenge you are working with a time series of stock prices. You are given historical records where the stock at time . . . Read more

Problem statement Given a string containing just the characters ( and ), find the length of the longest valid (well-formed) parentheses substring. Sample input (() Sample output 2 Sample input 1 )()()) Sample output 1 4 Solution We will solve this problem using a Stack. We are going to iterate . . . Read more

Problem statement Given an array of integers sorted in ascending order, find the starting and ending position of a given target value. If the target is not found in the array, return . Sample input [5, 7, 7, 8, 8, 10] 8 Sample output [3, 4] Solution class Solution(object): def . . . Read more