/**
 * @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;
};