skyline_problem_es6.js

let edificios = [
  [2, 9, 10], 
  [2, 4, 12],
  [3, 7, 15], 
  [5, 12, 12], 
  [15, 20, 10], 
  [19, 24, 8] 
]

function skyline(buildings){
  //this line get the most right wall of the buildings
  let maxRight = Math.max(...buildings.map(b => b[1]))
  
  //this function returns an array of buildings that appear on a given x position
  let buildingsOfX = x => buildings.filter(b => x >= b[0] && x <= b[1])
  
  //this variable holds the previous height as the loop goes on
  let heightPointer = 0
  
  //this variable will hold the points that outline the buildings
  let result = []
  
  //for each point from 1 to the outter left wall of buildings get the max height for that x
  for(let x=1; x<=maxRight+1; x++){
    
    //this function return all heights of all buildings from a given x point
    let heightsOfX = buildingsOfX(x).map(b => b[2])
    
    //this returns the height of the tallest building of the current x point
    let maxHeigth = heightsOfX.length? Math.max(...heightsOfX): 0
    
    //every time that the height changes we push previous and current point to the result array
    if(maxHeigth !== heightPointer){
      
      //push the previous and current points to the result array
      result.push([maxHeigth < heightPointer? x-1: x, heightPointer])
      result.push([maxHeigth < heightPointer? x-1: x, maxHeigth])
      
      //update the current height
      heightPointer = maxHeigth
    }
  }
  return result.map(x => x.join(',')).join('\n')
}

console.log(skyline(edificios))