Expand algebraic expression with distribution

##### Question Given an expression of lowercase letters, '+', '*', and parentheses with no '+' outside any parentheses, return an equivalent expression containing only '+' by fully expanding all products (e.g., (a+b)*(c+d) → ac+ad+bc+bd). Achieve O( 1) extra-space complexity. Follow-up: If '+' may appear outside parentheses (e.g., a+b*c*(d+e+f)+k+m*(g+h)+i), output the fully expanded '+'-only form (a+bcd+bce+bcf+k+mg+mh+i). Provide working code that may use O(n) extra space (e.g., a stack).