SOURCE

const getWaterAll=function(arr){
    let left = 0
    let right = arr.length -1
    let leftHeight = 0
    let rightHeight = 0
    let leftMaxHeight = 0
    let rightMaxHeight = 0
    let res = 0

    while(left<right){
        let leftHeight = arr[left]
        let rightHeight = arr[right]
        if(leftHeight<rightHeight){
            leftMaxHeight = Math.max(leftHeight,leftMaxHeight)
            res+=leftMaxHeight - leftHeight
            left++
        }else{
            rightMaxHeight = Math.max(rightHeight,rightMaxHeight)
            res+=rightMaxHeight - rightHeight
            right--
        }
    }
    console.log(res)
    return res
}

getWaterAll([0,1,0,2,1,0,1,3,2,1,2,1])