Infix expression evaluation

The basic idea ：

Define two stacks , Separate for storage Operands And Operator .

• Operands Direct access Operand stack
• For operators op,
  • If operator stack is empty , Direct stack
  • If it's left bracket (, Direct stack
  • If the top of stack operator is left parenthesis (, And op Is not ), Then push directly
  • if op The priority of the higher than Top of stack operators , Direct stack
  • if op The priority of the Below or equal to Top of stack operators , You need to calculate it before entering the stack
  • If it's a right parenthesis ), Calculation is required , Until I met (, here (...) The result of the formula in is calculated , take ( Kicked out
  • After each calculation , The situation in the stack will change , want Determine whether the calculation can continue （ That is, whether the current stack top operator still meets the conditions for calculation ）

Be careful ：(、) Brackets should be considered separately .

Some skills used ：

• use unordered_map Defines the priority of the operator , The priority of a given operator can be obtained .

unordered_map<char</