227. Basic Calculator II
Given a string s which represents an expression, evaluate this expression and return its value.
The integer division should truncate toward zero.
Note: You are not allowed to use any built-in function which evaluates strings as mathematical expressions, such as eval().
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| class Solution {
int nextVal(string& s, int& pos) {
int res = 0;
++pos;
for(; pos < s.length(); pos++) {
if(s[pos] == ' ') continue;
switch (s[pos]) {
case '+' :
case '-' :
case '*' :
case '/' : --pos; return res;
default: res = (res<<3) + (res<<1) + (s[pos]&0b1111);
}
}
return res;
}
public:
int calculate(string s) {
list<char> op;
list<int> num;
int val = 0;
for(int i = 0; i < s.length(); i++) {
if(s[i] == ' ') continue;
switch (s[i]) {
case '+' :
case '-' : num.push_back(val); op.push_back(s[i]); val = 0; break;
case '*' :
case '/' : val = s[i] == '*' ? val * nextVal(s, i) : val / nextVal(s, i); break;
default : val = (val<<3) + (val<<1) + (s[i]&0b1111);
}
}
num.push_back(val);
while(op.size()) {
int n1 = num.front(); num.pop_front();
int n2 = num.front(); num.pop_front();
int res = op.front() == '+' ? n1 + n2 : n1 - n2; op.pop_front();
num.push_front(res);
}
return num.front();
}
};
|
