progressive word combination of a string

I need to obtain a progressive word开发者_运维问答 combination of a string.

E.g. "this is string" Output: "this is string" "this is" "this string" "is string" "this" "is" "string"

Do you know similar algorithm? (I need it in php language) Thanks ;)

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This is a simple code solution to your problem. I concatenate each string recoursively to the remaining ones in the array.

$string = "this is a string";  
$strings = explode(' ', $string);

// print result
print_r(concat($strings, ""));

// delivers result as array
function concat(array $array, $base_string) {

    $results = array();
    $count = count($array);
    $b = 0;
    foreach ($array as $key => $elem){
        $new_string = $base_string . " " . $elem;
        $results[] = $new_string;
        $new_array = $array;

        unset($new_array[$key]);

        $results = array_merge($results, concat ($new_array, $new_string));

    }
    return $results;
}

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Check out eg. http://en.wikipedia.org/wiki/Permutation#Systematic_generation_of_all_permutations for an algorithm description.