Description
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.
Example
Example 1:
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| Input: "1 + 1"
Output: 2
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Example 2:
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| Input: " 2-1 + 2 "
Output: 3
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Example 3:
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| Input: "(1+(4+5+2)-3)+(6+8)"
Output: 23
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Note:
- You may assume that the given expression is always valid.
- Do not use the eval built-in library function.
Solution
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| class Solution {
public int calculate(String s) {
Stack<Integer> st = new Stack<>();
int res = 0;
int number = 0;
int sign = 1;
for (int i = 0; i < s.length(); i++){
char ch = s.charAt(i);
if (Character.isDigit(ch)){
number = number*10 + (int)(ch - '0');
} else if (ch == '+'){
res += sign * number;
number = 0;
sign = 1;
} else if (ch == '-'){
res += sign * number;
number = 0;
sign = -1;
} else if (ch == '('){
st.push(res);
st.push(sign);
res = 0;
sign = 1;
} else if (ch == ')'){
res += sign * number;
number = 0;
res *= st.pop();
res += st.pop();
}
}
if (number != 0) res += sign * number;
return res;
}
}
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