前序運算式
輸入 中序 輸出 前序
python
st = input()
level = {'+': 1, '-': 1, '*': 2, '/': 2}
output = []
stack = []
for char in reversed(st): # 反轉輸入字串，改為從右到左處理
if char.isalnum():
output.append(char)
elif char == ')':
stack.append(char)
elif char == '(':
while stack and stack[-1] != ')':
output.append(stack.pop())
stack.pop()
elif char in '+-*/':
while (stack and (stack[-1] in '+-*/') and (level[stack[-1]] > level[char])):
output.append(stack.pop())
stack.append(char)
while stack:
output.append(stack.pop())
print(''.join(reversed(output))) # 最終結果需要再次反轉以得到正確的前序表達式Contributors
lucashsu95