# Leetcode 125 Valid Palindrome ( C,C++,C#,Java,Javascript,Ruby)

Given a string `s`, determine if it is a palindrome, considering only alphanumeric characters and ignoring cases.

Example 1:

Example 2:

# Leetcode 657 Robot Return to Origin (C, C++, C#, Java, Javascript, Ruby)

There is a robot starting at position (0, 0), the origin, on a 2D plane. Given a sequence of its moves, judge if this robot ends up at (0, 0) after it completes its moves.

The move sequence is represented by a string, and the…

# Leetcode 200. Number of Islands

Given an `m x n` 2D binary grid `grid` which represents a map of `'1'`s (land) and `'0'`s (water), return the number of islands.

An island is surrounded by water and is formed by connecting adjacent lands horizontally or vertically. …

# Leecode 217 Contains Duplicate ( C++,C#,Java,Javascript,Ruby)

Contains Duplicate

Given an integer array nums, return true if any value appears at least twice in the array, and return false if every element is distinct.

Example 1:

Input: nums = [1,2,3,1] Output: true Example 2:

Input: nums = [1,2,3,4] Output: false Example 3:

Input: nums = [1,1,1,3,3,4,3,2,4,2] Output…

# Leetcode 34.Find First and Last Position of Element in Sorted Array

Given an array of integers `nums` sorted in ascending order, find the starting and ending position of a given `target` value.

If `target` is not found in the array, return `[-1, -1]`.

Follow up: Could you write an algorithm with `O(log n)` runtime complexity?

Example 1:

# Leecode 3 Longest Substring Without Repeating Characters

3. Longest Substring Without Repeating Characters

Given a string `s`, find the length of the longest substring without repeating characters.

Example 1:

Example 2:

# Leecode 949. Largest Time for Given Digits

Given an array `arr` of 4 digits, find the latest 24-hour time that can be made using each digit exactly once.

24-hour times are formatted as `"HH:MM"`, where `HH` is between `00` and `23`, and `MM` is between `00` and `59`. …

# Leetcode 1647 Minimum Deletions to Make Character Frequencies Unique

A string `s` is called good if there are no two different characters in `s` that have the same frequency.

Given a string `s`, return the minimum number of characters you need to delete to make `s` good.

The frequency of a character in a string is the number of…

## Chan Park

student at Flatiron School

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