LeetCode 42. Trapping Rain Water

/**
  * @param {number[]} height
  * @return {number}
  */
var trap   =  function (height ) {
    let maxLeft   =  0 ,maxRight   =  0 ;

    let left   =  0 ;
    let right   = height .length   - 1 ;
    let total   =  0 ;

    while  (left    <right ) {
        if  (height [left ]   <height [right ]) {
            if  (height [left ]  >maxLeft ) {
               maxLeft   = height [left ];
            } else  {
              total   += maxLeft  -height [left ]
            }
           left++ ;
        } else  {
            if  (height [right ]  >maxRight ) {
               maxRight   = height [right ];
            } else  {
              total   += maxRight  -height [right ]
            }
           right  -- ;
        }
    }
    return total ;
};