LeetCode刷题实战224：基本计算器
Given a string s representing an expression, implement a basic calculator to evaluate it.
示例
示例 1：
输入：s = "1 + 1"
输出：2
示例 2：
输入：s = " 2-1 + 2 "
输出：3
示例 3：
输入：s = "(1+(4+5+2)-3)+(6+8)"
输出：23
解题
class Solution {
public int evaluateExpr(Stack<Object> stack) {
int res = 0;
if (!stack.empty()) {
res = (int) stack.pop();
}
// Evaluate the expression till we get corresponding ')'
while (!stack.empty() && !((char) stack.peek() == ')')) {
char sign = (char) stack.pop();
if (sign == '+') {
res += (int) stack.pop();
} else {
res -= (int) stack.pop();
}
}
return res;
}
public int calculate(String s) {
int operand = 0;
int n = 0;
Stack<Object> stack = new Stack<Object>();
for (int i = s.length() - 1; i >= 0; i--) {
char ch = s.charAt(i);
if (Character.isDigit(ch)) {
// Forming the operand - in reverse order.
operand = (int) Math.pow(10, n) * (int) (ch - '0') + operand;
n += 1;
} else if (ch != ' ') {
if (n != 0) {
// Save the operand on the stack
// As we encounter some non-digit.
stack.push(operand);
n = 0;
operand = 0;
}
if (ch == '(') {
int res = evaluateExpr(stack);
stack.pop();
// Append the evaluated result to the stack.
// This result could be of a sub-expression within the parenthesis.
stack.push(res);
} else {
// For other non-digits just push onto the stack.
stack.push(ch);
}
}
}
//Push the last operand to stack, if any.
if (n != 0) {
stack.push(operand);
}
// Evaluate any left overs in the stack.
return evaluateExpr(stack);
}
}