题目：
题解：
func addOperators(num string, target int) (ans []string) {n := len(num)var backtrack func(expr []byte, i, res, mul int)backtrack = func(expr []byte, i, res, mul int) {if i == n {if res == target {ans = append(ans, string(expr))}return}signIndex := len(expr)if i > 0 {expr = append(expr, 0) // 占位，下面填充符号}// 枚举截取的数字长度（取多少位），注意数字可以是单个 0 但不能有前导零for j, val := i, 0; j < n && (j == i || num[i] != '0'); j++ {val = val*10 + int(num[j]-'0')expr = append(expr, num[j])if i == 0 { // 表达式开头不能添加符号backtrack(expr, j+1, val, val)} else { // 枚举符号expr[signIndex] = '+'; backtrack(expr, j+1, res+val, val)expr[signIndex] = '-'; backtrack(expr, j+1, res-val, -val)expr[signIndex] = '*'; backtrack(expr, j+1, res-mul+mul*val, mul*val)}}}backtrack(make([]byte, 0, n*2-1), 0, 0, 0)return
}
