[LeetCode] Basic Calculator II

Basic Calculator II

Implement a basic calculator to evaluate a simple expression string.

The expression string contains only non-negative integers, +, -, *, and / operators. 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

Note: Do not use the 

eval

 built-in
library function.

Credits:

Special thanks to @ts for adding this problem and creating all test cases.

解题思路：

同Basic Calculator I一样，只需改动转化成后缀表达式的代码。

class Solution {
public:
int calculate(string s) {
string postS = getPostString(s);    //获得后缀表达式
cout << postS;
stack<int> nums;
int len = postS.length();
int num1, num2;
for (int i = 0; i<len; i++){
switch (postS[i])
{
case '+':
num1 = nums.top();
nums.pop();
num2 = nums.top();
nums.pop();
num1 = num1 + num2;
nums.push(num1);
break;
case '-':
num1 = nums.top();
nums.pop();
num2 = nums.top();
nums.pop();
num1 = num2 - num1; //注意这里是num2减去num1
nums.push(num1);
break;
case '*':
num1 = nums.top();
nums.pop();
num2 = nums.top();
nums.pop();
num1 = num2 * num1;
nums.push(num1);
break;
case '/':
num1 = nums.top();
nums.pop();
num2 = nums.top();
nums.pop();
num1 = num2 / num1;
nums.push(num1);
break;
case '#':
break;
default:
int num = 0;
while (i < len && postS[i] >= '0' && postS[i] <= '9'){
num = num * 10 + (postS[i] - '0');
i++;
}
i--;
nums.push(num);
break;
}
}
return nums.top();
}

private:
string getPostString(string s){
string postS = "";

stack<char> op;
int len = s.length();
for (int i = 0; i<len; i++){
switch (s[i]){
case ' ':
break;
case '(':
op.push('(');
break;
case ')':
while (!op.empty() && op.top() != '('){
postS += op.top();
op.pop();
}
if (!op.empty() && op.top() == '('){
op.pop();
}
break;
case '*':
case '/':
while (!op.empty() && op.top() != '(' && op.top() != '+' && op.top() != '-'){    //这里体现出了*, /的优先级比+, -高
postS += op.top();
op.pop();
}
op.push(s[i]);
break;
case '+':
case '-':
while (!op.empty() && op.top() != '('){
postS += op.top();
op.pop();
}
op.push(s[i]);
break;
default:
while (i<len && s[i] >= '0'&&s[i] <= '9'){
postS += s[i];
i++;
}
postS += '#';   //分隔数字
i--;
break;
}
}
while (!op.empty()){
postS += op.top();
op.pop();
}

return postS;
}
};