Stacks Pseudocode

1procedure PUSH(S, x)
2  top ← top + 1
3  S[top] ← x
4end procedure

1procedure POP(S)
2  if top < 0 then
3    error "UNDERFLOW"
4  end if
5  x ← S[top]
6  top ← top − 1
7  return x
8end procedure

1procedure PEEK(S)
2  if top < 0 then
3    error "EMPTY"
4  end if
5  return S[top]
6end procedure

1procedure IS_BALANCED(expr)
2  create empty stack S
3  for each ch in expr do
4    if ch in '(', '[', '{' then
5      push(S, ch)
6    else if ch in ')', ']', '}' then
7      if S is empty then return false
8      open ← pop(S)
9      if NOT matches(open, ch) then return false
10    end if
11  end for
12  return S is empty
13end procedure