TrappedWater.js

var arr = [];

for(var i=100000;i>=1;i--) {
	arr.push(i);
}

for(var i=1;i<=100000;i++) {
	arr.push(i);
}

console.log(arr)
					


function calWater (arr) {
	var cur = null
  var next = null
  let waterTrapped = 0
	for(var start = 0; start < arr.length;) {
  	
  	var {index, sum} = findNextIndex(arr, start)
    if (index !== -1) {
    	waterTrapped +=  Math.min(arr[start], arr[index]) * (index - start-1) - sum
    	start = index
    } else {
    	start++
    }
  }
  return waterTrapped
}

function findNextIndex (arr, index) {
	var start = index + 1
  var sum = 0
  while(start < arr.length) {
  	if (arr[start] >= arr[index]) {
    	
    	return { index: start, sum}
    } else {
    	sum += arr[start]
    }
    start += 1
  
  }
  
  return {index: -1, sum}
	
}
console.log(calWater(arr))


/*
	
for each elment find next element which is equal or greatet than current hi

*/