Given an input string (s) and a pattern (p), implement regular
expression matching with support for '.' and '*'.'.' Matches any single character. '*' Matches zero or more of the
preceding element. The matching should cover the entire input string
(not partial).Note:
s could be empty and contains only lowercase letters a-z. p could be
empty and contains only lowercase letters a-z, and characters like .
or *. Example 1:Input: s = "aa" p = "a" Output: false Explanation: "a" does not match
the entire string "aa". Example 2:Input: s = "aa" p = "a" Output: true Explanation: '' means zero or
more of the precedeng element, 'a'. Therefore, by repeating 'a' once,
it becomes "aa". Example 3:Input: s = "ab" p = "." Output: true Explanation: "." means "zero or
more (*) of any character (.)". Example 4:Input: s = "aab" p = "cab" Output: true Explanation: c can be
repeated 0 times, a can be repeated 1 time. Therefore it matches
"aab". Example 5:Input: s = "mississippi" p = "misisp*." Output: false
比较难的一道题
第一思路是双指针解决，两个指针分别匹配中移动，如果最后都指向队尾，则匹配成功，利用贪心的思想，尽可能的往后匹配，最后判断两个串是否还有剩余，但没有考虑到一个情况，可能出现匹配多的情况，下面的情况就cover不了。
1111
1*1111
无法通过贪心解决的时候，就可以在无法决定的地方进行分支判断，按照正向的就是递归的思想，不要忘了保存状态。
正向可以通过递归方式解决
public boolean isMatch(String s, String p) { if(s.length()==0 && p.length()==0) return true; if(p.length()>=2 && p.charAt(1)=='*'){ if(p.length()>=1 && s.length()>=1 && (p.charAt(0)==s.charAt(0) || p.charAt(0)=='.')){ if(isMatch(s.substring(1),p)) return true; } if(isMatch(s,p.substring(2))) return true; } if(p.length()>=1 && s.length()>=1 && (p.charAt(0)==s.charAt(0) || p.charAt(0)=='.')){ if(isMatch(s.substring(1),p.substring(1))) return true; } return false;}但是有一个比较明显的优化点，重复计算了很多状态，可以用一个二维数组保存状态
int[][] dp;public boolean isMatch(String s, String p) { dp=new int[s.length()+1][p.length()+1]; dp[0][0]=1; return ref(s,p);}public boolean ref(String s, String p) { if(dp[s.length()][p.length()]!=0) return dp[s.length()][p.length()]==1; boolean first=p.length()>=1 && s.length()>=1 && (p.charAt(0)=='.' || p.charAt(0)==s.charAt(0)); if(p.length()>=2 && p.charAt(1)=='*'){ if(first){ if(ref(s.substring(1),p)){ dp[s.length()][p.length()]=1; return true; } } if(ref(s,p.substring(2))){ dp[s.length()][p.length()]=1; return true; } } if(first){ if(ref(s.substring(1),p.substring(1))){ dp[s.length()][p.length()]=1; return true; } } dp[s.length()][p.length()]=-1; return false;}