dp数组里存放LCS

#
# longest common subsequence
# @param s1 string字符串 the string
# @param s2 string字符串 the string
# @return string字符串
#
class Solution:
    def LCS(self , s1 , s2 ):
        # write code here
        m, n = len(s1), len(s2)
        dp = [['' for _ in range(n+1)] for _ in range(m+1)]
        for i in range(1, m+1):
            for j in range(1, n+1):               
                if s1[i-1] == s2[j-1]:
                    dp[i][j] = dp[i-1][j-1] + s2[j-1]
#                     print("dp[i][j]:", dp[i][j])
                else:
                    if len(dp[i][j-1]) > len(dp[i-1][j]):
                        dp[i][j] = dp[i][j-1]
                    else:
                        dp[i][j] = dp[i-1][j]
        if not dp[m][n]:
            return "-1"
        return dp[m][n]