Principles and applications of recursive calling of PHP functions

Principle of function recursion: function calls itself (self-reference). The parameters change each time it is called. Continue recursion until the recursion condition (stop condition) is met. Recursive application of functions: simplifying complex problems (decomposing them into sub-problems). Clean code (more elegant). Example: Calculate factorial (decomposed into products). Find the ancestors of a node in the tree (recursively search).

The principle and application of PHP function recursive calling

What is function recursion

Function recursion refers to a self-referential feature of a function calling itself. When a function is called within itself, it is called a recursive call.

The principle of recursion

1. The function calls itself.
2. In recursive calls, the parameters of the function will change.
3. The recursive process will continue until the recursive condition is reached.
4. After the recursion condition is met, the function will stop the recursion and return the result.

Advantages of recursion

• Solve complex problems: Recursion can break down complex problems into smaller sub-problems, thereby simplifying the solution.
• Code simplicity: Recursive code is usually more concise and elegant than non-recursive code.

Application Case

1. Calculate factorial

function factorial($number) {
  if ($number == 1) {
    return 1;
  } else {
    return $number * factorial($number - 1);
  }
}

echo factorial(5); // 输出: 120

2. Find the node in the tree Ancestor

class Node {
  public $data;
  public $children;
}

function findAncestors($node, $target) {
  if ($node->data == $target) {
    return [$node->data];
  } else {
    $ancestors = [];
    foreach ($node->children as $child) {
      $ancestors = array_merge($ancestors, findAncestors($child, $target));
    }
    if (!empty($ancestors)) {
      $ancestors[] = $node->data;
    }
    return $ancestors;
  }
}

$root = new Node(['data' => 'root']);
$node1 = new Node(['data' => 'node1']);
$node2 = new Node(['data' => 'node2']);
$node3 = new Node(['data' => 'node3']);
$root->children = [$node1, $node2];
$node2->children = [$node3];

$ancestors = findAncestors($root, 'node3');
var_dump($ancestors); // 输出: ['root', 'node2', 'node3']

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