Basic Calculator



题目描述：Implement a basic calculator to evaluate a simple expression string.

The expression string may contain open ( and closing parentheses ), the plus + or minus sign -, non-negative integers and empty spaces .

You may assume that the given expression is always valid.

Some examples:

"1 + 1" = 2
" 2-1 + 2 " = 3
"(1+(4+5+2)-3)+(6+8)" = 23

Note: Do not use the eval built-in library function.

解题思路：采用一个栈存储，使用边扫描字符串边入栈并计算的过程，当输入元素符合计算条件时，出栈并计算，计算后结果入栈。

Java代码实现：

public class Solution {
    public int calculate(String s) {
        Stack<String> stack=new Stack<String>();
        for(int i=0;i<s.length();i++){
            char ch=s.charAt(i);
            String strNum="";
            if(ch!=' '){
                if(ch=='+'||ch=='-'||ch=='('){
                    stack.push(String.valueOf(ch));
                }
                else if(ch>='0'&&ch<='9'){//保证多位数字
                    strNum=String.valueOf(ch);
                    while(i!=s.length()-1&&s.charAt(i+1)>='0'&&s.charAt(i+1)<='9'){
                        strNum+=s.charAt(++i);
                    }
                    while(!stack.isEmpty()){
                        String str=stack.peek();
                        if("(".equals(str)){
                            stack.push(strNum);
                            break;
                        }
                        if("+".equals(str)||"-".equals(str)){
                            String op=stack.pop();
                            if("+".equals(op)){
                                strNum=String.valueOf(Integer.valueOf(strNum)+Integer.valueOf(stack.pop()));
                            }
                            else if("-".equals(op)){
                                strNum=String.valueOf(Integer.valueOf(stack.pop())-Integer.valueOf(strNum));
                            }
                        }
                    }
                    if(stack.isEmpty()) stack.push(strNum);
                }
                else if(ch==')'){
                    strNum=stack.pop();
                    stack.pop();
                    while(!stack.isEmpty()){
                        String str=stack.peek();
                        if("(".equals(str)){
                            stack.push(strNum);
                            break;
                        }
                        if("+".equals(str)||"-".equals(str)){
                            String op=stack.pop();
                            if("+".equals(op)){
                                strNum=String.valueOf(Integer.valueOf(strNum)+Integer.valueOf(stack.pop()));
                            }
                            else if("-".equals(op)){
                                strNum=String.valueOf(Integer.valueOf(stack.pop())-Integer.valueOf(strNum));
                            }
                        }
                    }
                    if(stack.isEmpty()) stack.push(strNum);
                }
            }
                
        }
        return Integer.valueOf(stack.pop());
        
    }
}

原题地址：https://leetcode.com/problems/basic-calculator/

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