8. Z字形状变换

<?php
class Solution {
    private $ret = [];
    /**
     * LEETCODEISHIRINGLEETCODEISHIRING
     *
     * L   D   R   E   I   N
     * E  OE  II  ET  ES  IG
     * EC  IH  NL  CD  HR
     * T   S   G   O   I
     *
     * @param $s
     * @param $numRows
     * @return string
     */
    public function zigzag($s, $numRows) : string {
        if (strlen($s) <= 1 || $numRows == 1) return $s;
        $preI = -1;
        $sign = 'down';
        for ($i = 0; $i <= strlen($s); $i++) {
            if ($sign == 'up') {
                $preI = $preI - 1;
                $this->assign($preI, $s[$i]);
                if ($preI == 0 ) {
                    $sign = 'down';
                }
            } else {
                $preI = $preI + 1;
                $this->assign($preI, $s[$i]);
                if ($preI == $numRows - 1) {
                    $sign = 'up';
                }
            }
        }
        return implode("\n", array_map(function ($a) {return implode('', $a);}, $this->ret));
    }
    // 赋值
    private function assign($a, $value, $fill = ' ') {
        $this->ret[$a][] = $value;
        foreach ($this->ret as $k => $v) {
            if ($k != $a) {
                $this->ret[$k][] = $fill;
            }
        }
    }
}
$cls = new Solution();
$r = $cls->zigzag('LEETCODEISHIRINGLEETCODEISHIRINGLEETCODEISHIRINGLEETCODEISHIRINGLEETCODEISHIRING', 8);
print_r($r);