If characters are equal , execute recursive function for n - 1 , m - 1
if ( s1 [ n - 1 ] == s2 [ m - 1 ] ) { if ( dp [ n - 1 ] [ m - 1 ] == -1 ) { return dp [ n ] [ m ] = minDis ( s1 , s2 , n - 1 , m - 1 , dp ) ; } else return dp [ n ] [ m ] = dp [ n - 1 ] [ m - 1 ] ; }