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/**

* @param {string} s

* @return {number}

var calculate = function (s) {

var len = s.length;

var ret = 0, sign = 1, stack = [];

if (len === 0) {

return 0;

}

for (var i = 0; i < len; i++) {

if (!isNaN(parseInt(s[i]))) {

var sum = parseInt(s[i]);

while (i + 1 < len && !isNaN(parseInt(s[i + 1]))) {

sum = sum * 10 + parseInt(s[i + 1]);

i++;

}

ret += sum * sign;

} else if (s[i] === '+') {

sign = 1;

} else if (s[i] === '-') {

sign = -1;

} else if (s[i] === '(') {

stack.push(ret);

stack.push(sign);

ret = 0;

sign = 1;

} else if (s[i] === ')') {

ret = ret * stack.pop() + stack.pop();

}

return ret;

};