Twitter interview question

gistfile1.txt

function mySolution(walls){
    var heightLoc = {};
    var waterSum = 0;
    for(var x=0; x<walls.length; x++){
        for(var y=0; y<walls[x]; y++){
            var height = y+'';
            waterSum += height in heightLoc ? (x - heightLoc[height]) - 1  : 0;
            heightLoc[height] = x; // record last time we saw a wall of this height
        }
    } 
    return waterSum;
}
var walls = [2,5,1,3,1,2,1,7,1,5,1];
var myResult = mySolution(walls);