def solve(self):
queen_list = []
is_backtracking = False
backtracking_queen_pos = -1
try:
while True:
print "Trying", queen_list
# Check if all queens are placed
if len(queen_list) >= self._num_row:
print "Solution", queen_list
sys.exit(0)
# Select a position for next queen
is_backtracking = True
for pos in xrange(backtracking_queen_pos+1, self._num_row):
queen = Queen(pos)
# If a position is found, next iteration won't do backtracking.
if self._can_place(queen, queen_list):
queen_list.append(queen)
is_backtracking = False
backtracking_queen_pos = -1
break
# If no position could be found for next queen,
#   pop up last queen, so that we may continue with another position.
if is_backtracking:
backtracking_queen_pos = queen_list.pop().pos
except IndexError:
# When no solution could be found, at last queen_list would become [],
#   and queen_list.pop() would raise IndexError
print "No solution could be found!"