题解 | #路灯#

注意检查边界的coords[0]-0 和 l-coords[-1]

while True:
    try:
        n, l = list(map(int, input().split()))
        coords = list(map(int, input().split()))
    except:
        break

    coords = sorted(coords)
    ret = 0.00
    for i in range(1, len(coords)):
        ret = max(ret, (coords[i]-coords[i-1])/2)

    ret = max(ret, coords[0] - 0)
    ret = max(ret, l-coords[-1])              
    print("%.2f"%ret)