Groovy version of the "Calculate Rain Volume" algorithm.

CalculateRainVolume.groovy

def walls = [ 2, 5, 1, 2, 3, 4, 7, 7, 6 ]
println(calculateVolume(walls))

assert 10 == calculateVolume(walls)

def calculateVolume(land) {

    def leftMax = 0
    def rightMax = 0
    def left = 0
    def right = land.size() - 1
    def volume = 0
        
    while(left < right) {
        
        if(land[left] > leftMax) {
            leftMax = land[left]
        }
        
        if(land[right] > rightMax) {
            rightMax = land[right]
        }
        
        if (leftMax >= rightMax) {
            volume += rightMax - land[right]
            right--
        } else {
            volume += leftMax - land[left]
            left++
        }
    }
    
    return volume
}