Fixed Backwash issue in percolation

Compilation:  PASSED
Style:        PASSED
Findbugs:     No potential bugs found.
API:          PASSED

Correctness:  20/20 tests passed
Memory:       8/8 tests passed
Timing:       9/9 tests passed

Raw score: 100.00% [Correctness: 65%, Memory: 10%, Timing: 25%, Style:
0%]