224. Basic Calculator
224. Basic Calculator
题目
Given a string s
representing a valid expression, implement a basic calculator to evaluate it, and return the result of the evaluation.
Note: You are not allowed to use any built-in function which evaluates strings as mathematical expressions, such as eval()
.
Example 1:
Input: s = "1 + 1"
Output: 2
Example 2:
Input: s = " 2-1 + 2 "
Output: 3
Example 3:
Input: s = "(1+(4+5+2)-3)+(6+8)"
Output: 23
Constraints:
1 <= s.length <= 3 * 105
s
consists of digits,'+'
,'-'
,'('
,')'
, and' '
.s
represents a valid expression.'+'
is not used as a unary operation (i.e.,"+1"
and"+(2 + 3)"
is invalid).'-'
could be used as a unary operation (i.e.,"-1"
and"-(2 + 3)"
is valid).- There will be no two consecutive operators in the input.
- Every number and running calculation will fit in a signed 32-bit integer.
题目大意
实现一个基本的计算器来计算一个简单的字符串表达式的值。字符串表达式可以包含左括号 ( ,右括号 ),加号 + ,减号 -,非负整数和空格 。
解题思路
- 加减法计算器,其实就是一个去括号的过程,需要根据
+
-
和(
)
来判断数字的正负; - 遍历字符串,每当遇到
(
,就使用栈来保存括号前的运算符号; - 注意,负负得正的情况需要特殊处理,所以需要记录每次计算出来的符号;
- 遇到
)
,则出栈; - 遇到
+
-
,则用sign
来保存数字前的运算符号; - 每个数字的正负都取决于 栈顶和数字前的运算符号的乘积,即:
sum * sign * stack[stack.length - 1]
; - 将 数字 * 正负符号 累加起来即可;
代码
/**
* @param {string} s
* @return {number}
*/
/**
* @param {string} s
* @return {number}
*/
var calculate = function (s) {
let res = 0,
sum = 0,
sign = 1;
let stack = [1];
const isDigit = (str) => str <= "9" && str >= "0";
for (let i of s) {
if (isDigit(i)) sum = sum * 10 + Number(i);
else {
res += sum * sign * stack[stack.length - 1];
sum = 0;
if (i === "+") sign = 1;
else if (i === "-") sign = -1;
else if (i === "(") {
stack.push(stack[stack.length - 1] * sign);
sign = 1;
} else if (i === ")") stack.pop();
}
}
res += sum * sign;
return res;
};
相关题目
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- [227. 基本计算器 II](https://leetcode.com/problems/basic-calculator-ii)
- [241. 为运算表达式设计优先级](https://leetcode.com/problems/different-ways-to-add-parentheses)
- [282. 给表达式添加运算符](https://leetcode.com/problems/expression-add-operators)
- [🔒 Basic Calculator III](https://leetcode.com/problems/basic-calculator-iii)
- [2019. 解出数学表达式的学生分数](https://leetcode.com/problems/the-score-of-students-solving-math-expression)
- [2232. 向表达式添加括号后的最小结果](https://leetcode.com/problems/minimize-result-by-adding-parentheses-to-expression)