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basic calculator .java
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basic calculator .java
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class Solution {
public int calculate(String s) {
//evaluate expression recursively
return calculate(s.toCharArray(), 0)[0];
}
//evaluate an expression from the string, starting at idx
//0th index is result from expression.
//1st index is index after evaluting the expression.
int[] calculate(char[] s, int idx) {
int ret = 0;
int sign = 1;
//evaluate expression until end is found
while(idx < s.length && s[idx] != ')') {
//check to see if sign needs to be reversed.
for(; idx < s.length && s[idx] == ' '; idx++);
if(idx == s.length)
break;
if(s[idx] == '-') {
sign *= -1;
idx++;
}
for(; idx < s.length && s[idx] == ' '; idx++);
if(s[idx] == '(') {
//make recursive call to evaluate inner expression.
//add to result and advance index.
int[] tmp = calculate(s, idx+1);
ret += sign*tmp[0];
idx = tmp[1];
//assign starting sign for next op.
for(; idx < s.length && s[idx] == ' '; idx++);
if(idx < s.length && s[idx] != ')')
sign = s[idx++]=='+'?1:-1;
//read next number, and add it to the total.
}else {
//read next number and add it to the result.
int curr = 0;
while(idx < s.length && s[idx] >= '0' && s[idx] <= '9')
curr = curr*10 + s[idx++]-'0';
ret += sign*curr;
//assign starting sign for next op.
for(; idx < s.length && s[idx] == ' '; idx++);
if(idx < s.length && s[idx] != ')')
sign = s[idx++]=='+'?1:-1;
}
}
return new int[] {ret, idx+1};
}
}