05 August 2008
有加减乘除和括号
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class Solution {
public static int calculate(String s) {
if (s == null || s.length() == 0) return 0;
Stack<Integer> nums = new Stack<>(); // the stack that stores numbers
Stack<Character> ops = new Stack<>(); // the stack that stores operators (including parentheses)
int num = 0;
for (int i = 0; i < s.length(); i++) {
char c = s.charAt(i);
if (c == ' ') continue;
if (Character.isDigit(c)) {
num = c - '0';
// iteratively calculate each number
while (i < s.length() - 1 && Character.isDigit(s.charAt(i+1))) {
num = num * 10 + (s.charAt(i+1) - '0');
i++;
}
nums.push(num);
num = 0; // reset the number to 0 before next calculation
} else if (c == '(') {
ops.push(c);
} else if (c == ')') {
// do the math when we encounter a ')' until '('
while (ops.peek() != '(') nums.push(operation(ops.pop(), nums.pop(), nums.pop()));
ops.pop(); // get rid of '(' in the ops stack
} else if (c == '+' || c == '-' || c == '*' || c == '/') {
while (!ops.isEmpty() && precedence(c, ops.peek())) nums.push(operation(ops.pop(), nums.pop(),nums.pop()));
ops.push(c);
}
}
while (!ops.isEmpty()) {
nums.push(operation(ops.pop(), nums.pop(), nums.pop()));
}
return nums.pop();
}
private static int operation(char op, int b, int a) {
switch (op) {
case '+': return a + b;
case '-': return a - b;
case '*': return a * b;
case '/': return a / b; // assume b is not 0
}
return 0;
}
// helper function to check precedence of current operator and the uppermost operator in the ops stack
private static boolean precedence(char op1, char op2) {
if (op2 == '(' || op2 == ')') return false;
if ((op1 == '*' || op1 == '/') && (op2 == '+' || op2 == '-')) return false;
return true;
}
}