public int calculate(String s) {

int L = s.length();

int result = 0;

int start = 0;

boolean isNum = false;

boolean doPlus = true;

for (int i=0; i<L; i++) {

char c = s.charAt(i);

if (isDigit(c) && !isNum) {

start = i;

isNum = true;

continue;

}

if (isDigit(c)) continue;

if (isNum) {

result = doCal(s, result, start, i, doPlus);

isNum = false;

}

if (isPlus(c)) {

doPlus = true;

} else if (isMunis(c)) {

doPlus = false;

} else if (isOpen(c)) {

List<Integer> l = calculate(s, i+1, L);

result = doCal(result, l.get(0), doPlus);

i = l.get(1);

}

}

if (isNum) {

return doCal(s, result, start, L, doPlus);

}

return result;

}

private List<Integer> calculate(String s, int from, int L) {

int result = 0;

int start = from;

int to = from;

boolean isNum = false;

boolean doPlus = true;

for (int i=from; i<L; i++) {

char c = s.charAt(i);

if (isDigit(c) && !isNum) {

start = i;

isNum = true;

continue;

}

if (isDigit(c)) continue;

if (isNum) {

result = doCal(s, result, start, i, doPlus);

isNum = false;

}

if (isPlus(c)) {

doPlus = true;

} else if (isMunis(c)) {

doPlus = false;

} else if (isOpen(c)) {

List<Integer> l = calculate(s, i+1, L);

result = doCal(result, l.get(0), doPlus);

i = l.get(1);

} else {

to = i;

break;

}

}

if (isNum) {

return Arrays.asList(doCal(s, result, start, to, doPlus), to);

}

return Arrays.asList(result, to);

}

private int doCal(String s, int previous, int start, int end, boolean doPlus) {

int num = Integer.parseInt(s.substring(start, end));

return doCal(previous, num, doPlus);

}

private int doCal(int previous, int num, boolean doPlus) {

if (doPlus) {

return previous + num;

} else {

return previous - num;

}

}

private boolean isPlus(char c) {

return c == '+';

}

private boolean isMunis(char c) {

return c == '-';

}

private boolean isOpen(char c) {

return c == '(';

}

private boolean isClose(char c) {

return c == ')';

}

private boolean isDigit(char c) {

return c >= '0' && c <= '9';

}

private boolean isSpace(char c) {

return c == ' ';

}

/**

public int calculate2(String s) {

Stack<Integer> stack = new Stack<Integer>();

int result = 0;

int number = 0;

int sign = 1;

for(int i = 0; i < s.length(); i++){

char c = s.charAt(i);

if(Character.isDigit(c)){

number = 10 * number + (int)(c - '0');

}else if(c == '+'){

result += sign * number;

number = 0;

sign = 1;

}else if(c == '-'){

result += sign * number;

number = 0;

sign = -1;

}else if(c == '('){

//we push the result first, then sign;

stack.push(result);

stack.push(sign);

//reset the sign and result for the value in the parenthesis

sign = 1;

result = 0;

}else if(c == ')'){

result += sign * number;

number = 0;

result *= stack.pop(); //stack.pop() is the sign before the parenthesis

result += stack.pop(); //stack.pop() now is the result calculated before the parenthesis

}

}

if(number != 0) result += sign * number;

return result;

}

public int calculate3(String s) {

char[] chars = s.toCharArray();

int i = 0;

int N = chars.length;

int sign = 1;

Stack<String> stack = new Stack<>();

while (i < N) {

char ch = chars[i];

if (ch == ' ') {

i++;

} else if (ch == '+') {

sign = 1;

i++;

} else if (ch == '-') {

sign = -1;

i++;

} else if (ch == '(') {

stack.push((sign == 1 ? "+" : "-") + Character.toString(ch));

sign = 1;

i++;

} else if (ch == ')') {

int local = 0;

while (!stack.isEmpty() && !stack.peek().endsWith("(")) {

local += Integer.valueOf(stack.pop());

}

if (stack.isEmpty()) {

stack.push(Integer.toString(local));

} else {

String op = stack.pop();

if (op.startsWith("+")) {

stack.push(Integer.toString(local));

} else {

stack.push(Integer.toString(-local));

}

}

i++;

} else {

int j = getNum(chars, i);

stack.push((sign == 1 ? "+" : "-") + s.substring(i, j));

i = j;

}

}

int res = 0;

while (!stack.isEmpty()) res += Integer.valueOf(stack.pop());

return res;

}

private int getNum(char[] chars, int i) {

int j = i;

while (j < chars.length && Character.isDigit(chars[j])) j++;

return j;

}

/**

public int calculate4(String s) {

int pre = 0, curr = 0, sign = 1, op = 0, num = 0;

for (int i = 0; i < s.length(); i++) {

if (Character.isDigit(s.charAt(i))) {

num = num * 10 + (s.charAt(i) - '0');

if (i == s.length() - 1 || !Character.isDigit(s.charAt(i + 1))) {

curr = (op == 0 ? num : (op == 1 ? curr * num : curr / num));

}

} else if (s.charAt(i) == '*' || s.charAt(i) == '/') {

op = (s.charAt(i) == '*' ? 1 : -1);

num = 0;

} else if (s.charAt(i) == '+' || s.charAt(i) == '-') {

pre += sign * curr;

sign = (s.charAt(i) == '+' ? 1 : -1);

op = 0;

num = 0;

}

}

return pre + sign * curr;

}