1. [Initialize stack]
TOP <-- 1
S[TOP] <-- ‘(‘
2. [Initialize output string and rank count ]
POLISH <-- ‘ ‘
RANK <-- 0
3. [Get first input symbol]
NEXT<--NEXTCHAR (INFIX)
4. [Translate the infix expression ]
Repeat thru step 7 while NEXT != ‘ ‘
5. [Remove symbols with greater precedence from stack]
IF TOP < 1
Then write (‘INVALID’)
EXIT
Repeat while G (S[TOP]) > F(NEXT)
TEMP <-- POP (S, TOP)
POLISH <-- POLISH O TEMP
RANK <-- RANK + R(TEMP)
IF RANK <1
Then write ‘INVALID’)
EXIT
6. [Are there matching parentheses] IF G(S[TOP]) != F(NEXT)
Then call PUSH (S,TOP, NEXT)
Else POP (S,TOP)
7. [Get next symbol]
NEXT <- NEXTCHAR(INFIX)
8. [Is the expression valid]
IF TOP != 0 OR RANK != 1
Then write (‘INVALID ‘)
Else write (‘VALID ’)
