If st . Peek ( ) is present in the set s
if ( s . Contains ( st . Peek ( ) ) ) { s . Remove ( st . Peek ( ) ) ; st . Peek ( ) . left = node ; st . Pop ( ) ; } else { st . Peek ( ) . right = node ; } } st . Push ( node ) ; } while ( post [ p -- ] != init [ i ] && p >= 0 ) ; node = null ;