Add Backtracking

IIIT-A Lab (IAIN532C)/Backtracking/graph_colouring.py

@@ -0,0 +1,65 @@
+STATES = {\
+            0: "Western Australia", \
+            1: "Northern Territory", \
+            2: "South Australia", \
+            3: "Queensland", \
+            4: "New South Wales", \
+            5: "Victoria", \
+            6: "Tasmania"\
+        }
+
+GRAPH = {\
+            0: {1, 2}, \
+            1: {0, 2, 3}, \
+            2: {0, 1, 3, 4, 5}, \
+            3: {1, 2, 4}, \
+            4: {2, 3, 5}, \
+            5: {2, 4}, \
+            6: {}\
+        }
+
+COLOURS = ['RED', 'GREEN', 'BLUE']
+COLOURED_GRAPH = {}
+
+def check_valid(state, colour):
+    return not any(neighbour in COLOURED_GRAPH and COLOURED_GRAPH[neighbour] == colour \
+                    for neighbour in GRAPH[state])
+
+def set_colour(state, colour):
+    global COLOURED_GRAPH
+    COLOURED_GRAPH[state] = colour
+
+def colour_state(state):
+    for colour in range(3):
+        if check_valid(state, colour):
+            set_colour(state, colour)
+            return True
+    return False
+
+def change_previous_state(state):
+    for offset in range(3):
+        previous_state = state - 1
+        alternate_colour = (COLOURED_GRAPH[previous_state] + offset) % 3
+        if check_valid(previous_state, alternate_colour):
+            set_colour(previous_state, alternate_colour)
+            return True
+    return False
+
+def colour_graph():
+    state = 0
+    while state < len(STATES):
+        if colour_state(state):
+            state += 1
+        elif not change_previous_state(state):
+            state -= 1
+
+def print_coloured_graph():
+    for key, value in COLOURED_GRAPH.items():
+        print(STATES[key], ":", COLOURS[value])
+
+def main():
+    colour_graph()
+    print_coloured_graph()
+
+if __name__ == '__main__':
+    main()

IIIT-A Lab (IAIN532C)/Backtracking/n_queens.py

@@ -0,0 +1,23 @@
+BOARD_SIZE = 8
+
+def under_attack(col, queens):
+    return col in queens or \
+           any(abs(col - x) == len(queens) - i for i, x in enumerate(queens))
+
+def solve(n):
+    solutions = [[]]
+    for row in range(n):
+        solutions = (solution + [i+1]
+                     for solution in solutions
+                     for i in range(BOARD_SIZE)
+                     if not under_attack(i+1, solution))
+    return solutions
+
+def main():
+    answers = solve(BOARD_SIZE)
+    #first_answer = next(answers)
+    for answer in answers:
+        print(list(enumerate(answer, start=1)))
+
+if __name__ == '__main__':
+    main()