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
O(n) time, O(1) space
The operator * and / have higher priority over + and -. However, we might consider using + and – as a group separator. For example,
1 + 2 – 3 * 5 can be grouped as +1, +2, -(3*5). The answer would be to add the group values along the scan. At first, the sign is + by default, if we meet an operator, we store it, if we meet numbers, we store it as an intermediate number. We add the number to the final value if the current oper is + or -. Otherwise, we need to multiply/divide it with the previous value.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 | class Solution { public: int calculate(string s) { int r = 0; char oper = '+'; int tmp = 0; int i = 0; while (i < s.length()) { if (s[i] != ' ') { if (s[i] >= '0' && s[i] <= '9') { int num = 0; while (i < s.length() && s[i] >= '0' && s[i] <= '9') { num = num * 10 + s[i] - '0'; i ++; } if (oper == '+' || oper == '-') { r += tmp; tmp = num * (44 - oper); // 43 = +, 45 = - } else if (oper == '*') { tmp *= num; } else { tmp /= num; } continue; } else { oper = s[i]; } } i ++; } return r + tmp; } }; |
class Solution { public: int calculate(string s) { int r = 0; char oper = '+'; int tmp = 0; int i = 0; while (i < s.length()) { if (s[i] != ' ') { if (s[i] >= '0' && s[i] <= '9') { int num = 0; while (i < s.length() && s[i] >= '0' && s[i] <= '9') { num = num * 10 + s[i] - '0'; i ++; } if (oper == '+' || oper == '-') { r += tmp; tmp = num * (44 - oper); // 43 = +, 45 = - } else if (oper == '*') { tmp *= num; } else { tmp /= num; } continue; } else { oper = s[i]; } } i ++; } return r + tmp; } };
You can also build the reverse polish expression, which is a more generalized solution that can be used to evaluate the expression containing () brackets.
–EOF (The Ultimate Computing & Technology Blog) —
loading...
Last Post: Dynamic Programming - Integer Break
Next Post: How to Reverse Vowels of a String in C/C++?