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42. Trapping Rain Water
Given n non-negative integers representing an elevation map where the width of each bar is 1, compute how much water it can trap after raining.

Example 1:
Input: height = [0,1,0,2,1,0,1,3,2,1,2,1]
Output: 6
Explanation: The above elevation map (black section) is represented by array [0,1,0,2,1,0,1,3,2,1,2,1]. In this case, 6 units of rain water (blue section) are being trapped.

Example 2:

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Leetcode 347. Top K Frequent Elements
Given an integer array nums and an integer k, return the k most frequent elements. You may return the answer in any order.

 

Example 1:

Input: nums = [1,1,1,2,2,3], k = 2

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Leetcode 49. Group Anagrams
Given an array of strings strs, group the 
anagrams
 together. You can return the answer in any order.

Example 1:

Input: strs = ["eat","tea","tan","ate","nat","bat"]

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Leetcode 53 Maximum Subarray

# @param {Integer[]} nums
# @return {Integer}
def max_sub_array(nums)
    max = nums[0]
    sum = 0
    nums.each_with_index do |ele, i|
        sum += ele
        if sum > max 

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Leetcode 51 N queen

def solve_n_queens(n)
  @n = n
  @solutions = []

  board = Array.new(n) { Array.new(n, '.') }
  place_queens(board, 0)
  
  @solutions

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Leetcode 49 Group Anagram

# @param {String[]} strs
# @return {String[][]}

def group_anagrams(strs)
    sum_ascii_hash = {}
    strs.each_with_index do |str, i|
        sorted_aa = str.chars.sort.join
        sum_ascii_hash["#{sorted_aa}"] ? sum_ascii_hash["#{sorted_aa}"] << i : sum_ascii_hash["#{sorted_aa}"] = [i]

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Leetcode 48 Rotate image

# @param {Integer[][]} matrix
# @return {Void} Do not return anything, modify matrix in-place instead.
def rotate(matrix)
    # Just change row's to column, like [1,2,3] 1st row will become last column
    # Then 2nd row [4,5,6] will become 2nd column and so on
    #ignore: matrix.transpose.reverse
    n = matrix.length
    (0...n).each do |i|

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Leetcode 45 Jump game

# @param {Integer[]} nums
# @return {Integer}
def jump(nums)
    
    # Explanation:
    # Example: [6,4,3,1,1,5,4,3,2,3,1,9,7,4,3,2,1,1,6,2,1]
    # for each ele in loop
    # for current ele, check the next possible window with ele sized elements

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Leetcode 38: count and say

# @param {Integer} n
# @return {String}
def count_and_say(n)
    return "1" if n == 1
    str = count_and_say(n-1)
    str = str.split("").map {|ch| ch.to_i}
    #str.tally.map{|k,v| "#{v}#{k}" }.join("")
    len = str.length

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Leetcode 18: 4 sum hard


# def four_sum(nums, target)
#     nums.sort!
#     set = Set.new
#     output = []

#     (0..nums.length - 4).each do |i|
#         (i + 1..nums.length - 3).each do |j|