Basic Calculator II

题目：
 

Implement a basic calculator to evaluate a simple expression string.

The expression string contains only non-negative integers, +, -, *, / operators and empty spaces . The integer division should truncate toward zero.

You may assume that the given expression is always valid.

Some examples:

"3+2*2" = 7
" 3/2 " = 1
" 3+5 / 2 " = 5
基本思路：     把减法当成负数入栈，把正数的数值全部入栈，遇到*/运算先计算后将结果入栈，最后栈中的所有数值依次取出加法运算

 

 1 public class Solution {
 2     public int calculate(String s) {
 3         
 4         if (s != null && s.length() != 0) {
 5             Stack<String> stack = new Stack<>();
 6             int result = 0;
 7             int i = 0;
 8             while (i < s.length()) {
 9                 if(s.charAt(i) == ' ' || s.charAt(i) == '+'){
10                     i++;
11                 }
12                 else {
13                     char tempChar = s.charAt(i);
14                     StringBuffer buffer = new StringBuffer();
15                         if(tempChar == '*' || tempChar == '/'){
16                         int tempResult;
17                         String firstVal = stack.pop();
18                         StringBuffer secondVal = new StringBuffer();
19                         int j = i+1;
20                         while(s.charAt(j) == ' '){
21                             
22                             j++;
23                         }
24                         secondVal.append(s.charAt(j));
25                         while(j+1 != s.length() && s.charAt(j+1) > 47 && s.charAt(j+1) < 58){
26                             secondVal.append(s.charAt(++j));
27                         }
28                         if(tempChar == '*')
29                             tempResult = Integer.parseInt(firstVal) * Integer.parseInt(secondVal.toString()) ;
30                         else {
31                             tempResult = Integer.parseInt(firstVal) / Integer.parseInt(secondVal.toString()) ;
32                         }
33                         stack.push(Integer.toString(tempResult));
34                         if(j>i){
35                             i=j;
36                             i++;
37                         }
38                             
39                         else {
40                             i++;
41                         }
42                     }
43                     
44                     else{
45                         buffer.append(s.charAt(i));
46                         if(i < (s.length()-1)){
47                             int j = i+1;
48                             while(j < s.length() && s.charAt(j) == ' '){
49                                 
50                                 j++;
51                             }
52                             while(j != s.length() && s.charAt(j) > 47 && s.charAt(j) < 58){                    
53                                 buffer.append(s.charAt(j++));
54                             }
55                             if(j>i){
56                                 i=j;
57                             }
58                                 
59                             else {
60                                 i++;
61                             }
62                         }else{
63                             i++;
64                         }
65                         stack.push(buffer.toString());
66                     }
67                 }
68             }
69             while(!stack.isEmpty() && stack.size() != 1){
70                 String secondVal = stack.pop();
71                 String firstVal = stack.pop();
72                 int tempResult = 0;
73                 tempResult = Integer.parseInt(firstVal) + Integer.parseInt(secondVal.toString());
74                 stack.push(Integer.toString(tempResult));
75             }
76             return Integer.parseInt(stack.pop());
77         }
78         return 0;
79         
80     
81     }
82 }

 

posted @ 2016-11-23 10:48  music180  阅读(124)  评论(0)  编辑  收藏  举报