38. 外观数列

<?php
class Solution {
    /**
     * @param Integer $n
     * @return String
     */
    public function countAndSay($n) {
        if ($n < 1) {
            return '';
        }
        if ($n == 1) {
            return '1';
        }
        $str = strval($this->countAndSay($n - 1));
        $lastS = substr($str, 0, 1);
        $countMap = [];
        $newStr = '';
        for ($i = 0; $i < strlen($str); $i++) {
            if ($str[$i] != $lastS) {
                $newStr .= $countMap[$lastS] . $lastS;
                unset($countMap[$lastS]);
                $lastS = $str[$i];
            }
            if (isset($countMap[$lastS])) {
                $countMap[$lastS]++;
            } else {
                $countMap[$lastS] = 1;
            }
        }
        $newStr .= $countMap[$lastS] . $lastS;
        return $newStr;
    }
}
$n = 10;
$cls = new Solution();
$str = $cls->countAndSay($n);
echo $str;