LeetCode Basic Calculator Stack (golang)

Promblem

Analysis Process
1.Define two stacks，One is the numeric stack and the other is the symbol stack
2.Define symbol priority
3.Same-level symbols can pop up, low-level symbols can pop up high-level symbols (except open bracket)
4.The numeric stack and symbol stack are pushed in order, and the symbol stack determines the operation order according to the symbol level

Code

func calculate(s string) int {
    num := 0  // Extract the Numbers in s
	res := 0  // Returns the result of the calculation
	sign := 1 //record operation symbol
	stack := make([]int, 0, len(s))
	for i := 0; i < len(s); i++ {
		switch s[i] {
		case '0', '1', '2', '3', '4', '5', '6', '7', '8', '9':
			num = 0
			for ; i < len(s) && s[i] >= '0' && s[i] <= '9'; i++ {
				// multi-digit Numbers
				num = 10*num + int(s[i]-'0')
			}
			res += sign * num
			i--
		case '+':
			sign = 1
		case '-':
			sign = -1
		case '(':
// When open bracket is encountered, push the current res and sign onto the stack and save the current runtime environment
			stack = append(stack, res, sign)
			res = 0
			sign = 1
		case ')':
			// when  close bracket is encountered,pop the stack
			// sign is the opreation symbol before ')'which matches '('
			sign = stack[len(stack)-1]
			// temp is the result of calculation before the sign
			temp := stack[len(stack)-2]
			stack = stack[:len(stack)-2]
			// '(' and ')' result of the calculation
			//          ↓
			res = sign*res + temp
		}
	}
	return res
}