Basic Calculator
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.
https://leetcode.com/problems/basic-calculator/
这这这不是我每天都为之纠结的后缀表达式吗。
题目比较简单只有加减法，而且没有算上负数。
我赌5毛，将来会有一道hard就是这题带上负数，小数，大小括号，乘除法之类，名字我都起好了Basic Calculator II。
2个栈，第一个是放运算结果（或者中间运算结果）的栈resultStack，第二个是放运算符的栈opStack。
举栗子：1+2
1 --> resultStack : [1], opStack : []
+ --> resultStack : [1], opStack : [+]
2 --> resultStack : [1,2], opStack : [+]
压缩二元运算操作compress_2operators() --> resultStack : [3], opStack : []
结束
举栗子2： 3-(2-1)
3 --> resultStack : [3], opStack : []
- --> resultStack : [3], opStack : [-]
( --> resultStack : [3], opStack : [-,(]
2 --> resultStack : [3,2], opStack : [-,(]
- --> resultStack : [3,2], opStack : [-,(,-]
1 --> resultStack : [3,2,1], opStack : [-,(,-]
) --> 压缩括号操作compress_bracket()，中间会调用compress_2operators() --> resultStack : [3,1], opStack : [-]
压缩二元运算操作compress_2operators() --> resultStack : [2], opStack : []
结束
如果要加上乘除法，需要修改一下正则，加上乘除法和加减法的优先级。
1 /**
2 * @param {string} s
3 * @return {number}
4 */
5 var calculate = function(s) {
6 var resultStack = [];
7 var opStack = [];
8 var temp = "";
9 for(var i = 0; i < s.length; i++){
10 var ch = s[i];
11 if(/^(\+|\-)$/.test(ch)){ // + -
12 compress_2operators();
13 opStack.push(ch);
14 } else if (ch === ‘(‘){
15 opStack.push(ch);
16 } else if (ch === ‘)‘){
17 compress_bracket();
18 } else if (/^[0-9]$/.test(ch)){
19 temp += ch;
20 }
21
22 if(s[i + 1] && /^(\+|\-|\(|\))$/.test(s[i + 1])){ // + - ( )
23 if(temp !== ""){
24 resultStack.push(parseInt(temp));
25 temp = "";
26 }
27 }
28 }
29 if(temp !== ""){
30 resultStack.push(parseInt(temp));
31 temp = "";
32 }
33 compress_2operators();
34 return resultStack.pop();
35
36 function compress_bracket(){
37 while(opStack[opStack.length - 1] !== ‘(‘){
38 compress_2operators();
39 }
40 opStack.pop(); //(
41 }
42 function compress_2operators(){
43 while(/^(\+|\-)$/.test(opStack[opStack.length - 1])){ // + -
44 var op = opStack.pop();
45 var right = resultStack.pop();
46 var left = resultStack.pop();
47 if(op === ‘+‘){
48 resultStack.push(left + right);
49 }else{
50 resultStack.push(left - right);
51 }
52 }
53 }
54 };