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# All posts in Algorithms

### Permutations challenge – Iterative solution

Problem statement Given a list, nums, of distinct numbers, return all possible permutations. Sample input

Sample output

Solution In another post we showed how this problem can be solved using a recursive technique by swapping. We stumbled upon a nice iterative solution that we thought might be worth . . . Read more

### Permutations challenge

Problem statement Given a list, nums, of distinct numbers, return all possible permutations. Sample input

Sample output

Solution We are using a technique called permutations by swapping. Full code

### Rotate Matrix / Image challenge

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

### Time series queries challenge

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

### Longest valid parentheses challenge

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

Sample input 1

Sample output 1

Solution We will solve this problem using a Stack. We are going to iterate . . . Read more

### Search for a range challenge

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

Sample output

Solution

### Trapping rain water challenge

Problem statement Given n non-negative integers representing an elevation map where the width of each bar is 1, compute how much water it is able to trap after raining. Sample input

Sample output

Solution We can solve this problem in time by iterating over the array twice. Once . . . Read more

### Letter combinations of a phone number challenge

Problem statement Given a digit string, return all possible letter combinations that the number could represent. A mapping of digit to letters (just like on the telephone buttons) is given below. Sample input

Sample output

Solution

### Search in rotated sorted array challenge

Problem statement Suppose an array sorted in ascending order is rotated at some pivot unknown to you beforehand. (i.e., might become ). You are given a target value to search. If found in the array return its index, otherwise return . You may assume no duplicate exists in the array. . . . Read more

### Zigzag conversion challenge

Problem statement The string PAYPALISHIRING is written in a zigzag pattern on a given number of rows like this:

And then read line by line: PAHNAPLSIIGYIR. Write the code that will take a string and make this conversion given a number of rows: Sample input

Sample output