深さ優先探索

DfsPractice.php

<?php
    class DfsPractice{
        private $size  = 2;
        private $count = 0;
        
        public function exec(){
            $this->dfs(0, 0);
            echo sprintf("全部で%d通り", $this->count);
        }
        
        private function dfs(int $x, int $y){
            if ($x == $this->size && $y == $this->size){
                echo "カウント追加: size:" . $this->size . " x:" . $x . " y:" . $y . "\n";
                $this->count++;
                return;
            }
            
            if ($y<$this->size){
                $this->dfs($x, $y + 1);
            }
            
            if ($x<$this->size){
                $this->dfs($x + 1, $y);
            }
        }
    }

    $d = new DfsPractice();
    $d->exec();