If L is 2 , then we just need to compare two characters . Else need to check two corner characters and value of P [ i + 1 ] [ j - 1 ]
if ( $ L == 2 ) $ P [ $ i ] [ $ j ] = ( $ str [ $ i ] == $ str [ $ j ] ) ; else $ P [ $ i ] [ $ j ] = ( $ str [ $ i ] == $ str [ $ j ] ) && $ P [ $ i + 1 ] [ $ j - 1 ] ;