(Algorithms in Javascript) Leetcode 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 is able to trap after raining.

leetcode-42-trapping-rain-water.js

const trap = (number) => {
  let len = number.length;
  if(!len) return 0; 
  let result = 0;
  let maxLeft = [];
  maxLeft[0] = number[0];
  let maxRight = [];
  maxRight[len-1] = number[len-1];
  
  for(let i=1; i<len; i++) {
    maxLeft[i] = Math.max(number[i], maxLeft[i-1]);
  }
  for(let i=len-2; i>=0; i--){
    maxRight[i] = Math.max(number[i], maxRight[i+1]);
  }
  for(let i=0; i<len; i++){
    result += Math.min(maxLeft[i], maxRight[i]) - number[i];
  }
  
  return result;
};