calculate extra spaces in a single line . The value extra [ i ] [ j ] indicates extra spaces if words from word number i to j are placed in a single line
for ( i = 1 ; i <= n ; i ++ ) { extras [ i ] [ i ] = M - l [ i - 1 ] ; for ( j = i + 1 ; j <= n ; j ++ ) extras [ i ] [ j ] = extras [ i ] [ j - 1 ] - l [ j - 1 ] - 1 ; }