直接上代碼

public static class InfixToPostfix
{
    private static List<char> highOperator = new List<char> { '*', '/' };
    private static List<char> lowOperator = new List<char> { '+', '-' };

    public static string Method(string str)
    {
        var stack = new Stack<char>();
        var ans = new StringBuilder();
        foreach (char item in str)
        {
            if (item >= 48 && item <= 57)
            {
                ans.Append(item);
            }
            else if (item == '(')
            {
                stack.Push(item);
            }
            else if (item == ')')
            {
                while (stack.Count > 0)
                {
                    var preOpe = stack.Pop();
                    if (preOpe == '(')
                    {
                        break;
                    }
                    ans.Append(preOpe);
                }
            }
            else // 操作符
            {
                if (stack.Count != 0)
                {
                    var preOpe = stack.Peek();
                    // 棧頂操作符優(yōu)先級大于等于當(dāng)前操作符優(yōu)先級
                    if ((highOperator.Contains(preOpe) && highOperator.Contains(item))
                        || (highOperator.Contains(preOpe) && lowOperator.Contains(item)))
                    {
                        ans.Append(stack.Pop());
                    }
                }

                stack.Push(item);
            }
        }

        while (stack.Count > 0)
        {
            ans.Append(stack.Pop());
        }

        return ans.ToString();
    }
}