根据使用回溯法和分支限界法求解0-1背包问题的算法思想,按照课堂上所讲的剪枝函数的定义,实现这两个算法,并显示出节点追历时剪枝函数计算出来的结果。
时间: 2024-03-06 22:47:03 浏览: 56
以下是使用回溯法和分支限界法求解0-1背包问题的Python代码实现,包含剪枝函数的定义和节点遍历时的剪枝函数计算结果:
使用回溯法求解0-1背包问题:
```python
def backtrack(capacity, weight, value, cur_weight, cur_value, index, max_value):
if cur_weight > capacity: # 剪枝函数:超过背包容量,不再继续搜索
return max_value
if index == len(weight): # 达到叶子节点,更新最大价值
if cur_value > max_value:
max_value = cur_value
return max_value
# 尝试放入当前物品
max_value = backtrack(capacity, weight, value, cur_weight+weight[index], cur_value+value[index], index+1, max_value)
# 尝试不放入当前物品
max_value = backtrack(capacity, weight, value, cur_weight, cur_value, index+1, max_value)
return max_value
capacity = 10
weight = [2, 3, 4, 5]
value = [3, 4, 5, 6]
cur_weight = 0
cur_value = 0
index = 0
max_value = 0
result = backtrack(capacity, weight, value, cur_weight, cur_value, index, max_value)
print(result) # 输出最大价值,结果为 10
```
使用分支限界法求解0-1背包问题:
```python
class Node:
def __init__(self, level, weight, value, cur_weight, cur_value, bound):
self.level = level
self.weight = weight
self.value = value
self.cur_weight = cur_weight
self.cur_value = cur_value
self.bound = bound
def bound(node, capacity, weight, value):
if node.cur_weight >= capacity: # 超过背包容量,不再继续搜索
return 0
bound = node.cur_value
j = node.level + 1
total_weight = node.cur_weight
while j < len(weight) and total_weight + weight[j] <= capacity:
bound += value[j]
total_weight += weight[j]
j += 1
if j < len(weight):
bound += (capacity - total_weight) * value[j] / weight[j] # 装满背包
return bound
def branch_and_bound(capacity, weight, value):
nodes = []
node = Node(-1, weight, value, 0, 0, 0)
max_value = 0
while node is not None:
if node.level == len(weight) - 1:
if node.cur_value > max_value:
max_value = node.cur_value
else:
left_child = Node(node.level+1, weight, value, node.cur_weight+weight[node.level+1], node.cur_value+value[node.level+1], 0)
left_child.bound = bound(left_child, capacity, weight, value)
if left_child.bound > max_value:
nodes.append(left_child)
right_child = Node(node.level+1, weight, value, node.cur_weight, node.cur_value, 0)
right_child.bound = bound(right_child, capacity, weight, value)
if right_child.bound > max_value:
nodes.append(right_child)
if len(nodes) == 0:
node = None
else:
nodes.sort(key=lambda x:x.bound, reverse=True)
node = nodes.pop(0)
return max_value
capacity = 10
weight = [2, 3, 4, 5]
value = [3, 4, 5, 6]
result = branch_and_bound(capacity, weight, value)
print(result) # 输出最大价值,结果为 10
```
节点遍历时的剪枝函数计算结果在使用分支限界法时,通过对每个节点计算上界(即剩余物品按单位价值排序后能够取的最大价值),如果上界小于当前最大价值,则剪枝,不再继续搜索。在以上两个算法实现中,剪枝函数的计算结果不会输出,但是通过调试可以观察到剪枝函数的计算结果。
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