<?php


/**
 * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
 * longest common subsequence
 * @param s1 string字符串 the string
 * @param s2 string字符串 the string
 * @return string字符串
 */
function LCS( $s1 ,  $s2 )
{
    // write code here
    $len1 = strlen($s1);
     $len2 = strlen($s2);
    if( $len1 == 0 || $len2 == 0  ){
        return -1;
    }
    for($i = 0; $i < $len1+1; $i++){
            for($j = 0; $j < $len2+1; $j++){
                if($j==0||$i==0){
                    $dp[$i][$j] =0;
                    continue;
                }
               
                if($s1[$i-1] == $s2[$j-1]){
                     $dp[$i][$j] =$dp[$i-1][$j-1]+1;
                }else{
                    $dp[$i][$j] = max($dp[$i-1][$j],$dp[$i][$j-1]);
                }
            }
    }
   
    $re = '';
    while($len1 !=0 && $len2!=0){
       
        if($s1[$len1-1] == $s2[$len2-1]  ){
            $re = $re.$s1[$len1-1] ;
            --$len1;
            --$len2;
        }else{
            if($dp[$len1-1][$len2] > $dp[$len1][$len2-1]){
                $len1--;
            }else{
                $len2--;
            }
        }
    }
    return strrev($re);
    
    
    
    
    
}