Contents

Backtracking 

Backtracking is a technique used to build up to a solution to a problem incrementally. These “partial solutions” can be phrased in terms of a sequence of decisions. Once it is determined that a “partial solution” is not viable (i.e. no subsequent decision can convert it into a solution) then the backtracking algorithm retraces its step to the last viable “partial solution” and tries again.

Visualizing the decisions as a tree, backtracking has many of the same properties of depth-first search. The difference is that depth-first search will guarantee that it visits every node until it finds a solution, whereas backtracking doesn’t visit branches that are not viable.

Because backtracking breaks problems into smaller subproblems, it is often combined with dynamic programming, or a divide-and-conquer approach.

Concepts

• State: A “partial solution” to the problem.
• Rejection criterion: A function that rejects a partial solution as “unrecoverable”. i. e. no sequence of subsequent decisions can turn this state into a solution. Without rejection criteria, backtracking is the same as depth-first search.
• Viable: A state that may still lead to a solution. This reflects what is known “at this stage”. All states that fail the rejection criteria are not viable. If we have a state that is initially viable, and find that all paths to leaves eventually terminate at a state that fail the rejection criteria, the state will become non-viable.
• Heuristic: Backtracking will (eventually) look at all the different viable nodes by recursively applying all the possible decisions. A heuristic is a quick way of ordering which decisions are likely to be the best ones at each stage, so that they get evaluated first. The goal is to find a solution early (if one exists).
• Pruning: The concept of determining that there are no nodes / states along a particular branch, so it is not worth visiting those nodes.

Generalised Algorithm

In the algorithms below, it is assumed that each decision is irreversible, so there is only one path to each state. If modeling a game like chess, you would have to be more careful, as it is possible to arrive at the same state multiple different ways. If it is possible to reach each state multiple ways, then we would also need to keep track of which states we had already visited.

A recursive implementation of a backtracking algorithm takes the general form

def doBacktrack( current ):
if current is a solution:
return current
for each decision d from current:
new_state <- state obtained from current by making decision d
if new_state is viable:
sol <- doBacktrack(new_state)
if sol is not None:
return sol
# indicate there is no solution
return None    

Iterative solution

def doBacktrackIterative(start):
stack = [start]

while len(stack) > 0:
current = stack.pop()

if current is a solution:
yield current

for decision in possible_decisions(current):
new_state <- state obtained from current by making decision d
if is_viable(new_state):
stack.push(new_state)

Example: n queens

Placing n queens on an n x n chess board so no queen can be taken

We know that we need one queen in every row, and one queen in every column.

• Initial state: An empty board
• The decision: The decision at stage i is which column to put the queen on row i into.
• Rejection criterion: Reject all configurations that do not allow any queen on any of the lower rows. (Adding more queens to the board will not prevent this row from being blocked.)

Here are the states that the backtracking algorithm explores for a 4x4 board. Note that naively there are 4! = 24 complete configurations with 4 queens on the board, if we place the queens row-by-row, and avoid reusing a column that is already used. With backtracking, many configurations are eliminated early. This algorithm shows all solutions, instead of stopping at the first one found. # solution

def _is_solution(current, n) -> "bool":
if len(current) != n: return False
return _is_viable(current, n)

def _is_viable(current, n) -> "bool":
"for each placed queen check if it hits another"
# by design we do not need to check columns
# rows
if len(current) != len(set(current)):
return False
# diagonals
for i, pos in enumerate(current):
for j in range(len(current)):
if i != j:
if current[j] == pos + (j-i): return False
if current[j] == pos - (j-i): return False
return True

def _possible_decisions(current, n) -> "list":
return list(range(1,n+1))

def _find_queen_positions(n):
"generator"
stack = [[]]
while len(stack)>0:
current = stack.pop()
if _is_solution(current, n):
yield current
for d in _possible_decisions(current, n):
new_state = current.copy()
new_state.append(d)
if _is_viable(new_state, n):
stack.append(new_state)

def nQueens(n):
if n == 0: return [[]]
if n == 1: return []
return sorted( list( _find_queen_positions(n) ) )

Example: sum subsets

Given a sorted array of integers arr and an integer num, find all possible unique subsets of arr that add up to num. Both the array of subsets and the subsets themselves should be sorted in lexicographical order.

def sumSubsets(arr, num):
result = set()

if target == 0:
elif arr_i >= len(arr) or target < 0:
return
else:
n = arr[arr_i]

return sorted(list(result))

Dynamic Programming 

Break down problem into reoccurring smaller problems. Then save results for the reoccurring problem, so it would not be required to solve the same problem more than once. Two common approaches include:

• Bottom-up strategy (iteration): In this strategy, you start solving from the smallest subproblem up towards the original problem. If you’re doing this, you’ll solve all of the subproblems, thereby solving the larger problem. The cache in bottom-up solutions is usually an array (either one or multi-dimensional).
• Top-down strategy (recursion): Start solving the identified subproblems. If a subproblem hasn’t been solved yet, solve it and cache the answer; if it has been solved, return the cached answer. This is called memoization. The cache in top-down solutions is usually a hash table, binary search tree, or other dynamic data structure.

Generalised algorithm

1. Find a recursive or iterative solution to the problem first (although usually these will have bad run times).
2. This helps you identify the subproblems, which are “smaller” instances of the original problem.
3. Determine whether the subproblems are overlapping. If they’re overlapping, a recursive algorithm will be solving the same subproblem over and over. (If they’re not overlapping and the recursive algorithm generates new subproblems each time it runs, DP isn’t a good solution; try a divide-and-conquer solution like merge sort or quick sort instead.)
4. Store/cache the results for every subproblem.
5. Once the subproblems have solutions, the original problem is easy to solve!

Example: filling blocks

You have a block with the dimensions 4 × n. Find the number of different ways you can fill this block with smaller blocks that have the dimensions 1 × 2. You are allowed to rotate the smaller blocks.

For n = 1, the output should be fillingBlocks(n) = 1. For n = 4, the output should be fillingBlocks(n) = 36.

The reoccurring subproblem is along the n axis. For each row there are number of combinations ($2^4=16$ in total). We can compute all the possible transitions using the following code

def state_to_int(state):
out = 0
for i, x in enumerate(state[::-1]):
out += x * 2**i
return out

def from_int_to_state(x):
out = tuple()
for i in range(4):
out = out + (x % 2 ,)
x = x // 2
return out

def find_options(state) -> "list of states":
out = []
stack = [tuple()]
while len(stack) > 0:
current = stack.pop()
if len(current) == 4:
out.append(current)
else:
i = len(current)
if state[i]:
stack.append(current + (0,))
else:
stack.append(current + (1,))
if len(current) < 3 and state[i+1] == 0:
stack.append(current + (0,0,))
return out

def find_vec_transitions() -> "dict[int: list[int]]":
out = {}
for i in range(16):
res = find_options( from_int_to_state(i) )
out[i] = list(map(state_to_int, res))
return out

Where the output gives us the transition from layer n to n+1:

find_vec_transitions()
{0: [0, 3, 9, 12, 15],
1: [1, 4, 7],
2: [8, 11],
3: [0, 3],
4: [1, 13],
5: ,
6: ,
7: ,
8: [2, 8, 14],
9: [0, 6],
10: ,
11: ,
12: [0, 12],
13: ,
14: ,
15:   }

Observe that a lot of the states are unreachable. There are only 6 states that can be reached with a start at 0 state: {0, 3, 6, 9, 12, 15}. Not need to count the states.