LeetCode 125. Valid Palindrome

Posted 2020-07-28 浩然

https://leetcode.com/problems/valid-palindrome/

 

Given a string, determine if it is a palindrome, considering only alphanumeric characters and ignoring cases.

For example,
"A man, a plan, a canal: Panama" is a palindrome.
"race a car" is not a palindrome.

Note:
Have you consider that the string might be empty? This is a good question to ask during an interview.

For the purpose of this problem, we define empty string as valid palindrome.

 

• 字符串题。两种解法，一种是直接把string翻转比较，另一种是传统的首尾两个指针移动比较。
• 注意对指针-- end，end不能为size_t类型, 否则一旦被减为-1则会被转变为INT_MAX，程序进入死循环。
• 学习了string相关STL算法和函数，往string里添加char用push_back。
• toupper - C++ Reference
  • 　　http://www.cplusplus.com/reference/cctype/toupper/
• isalnum - C++ Reference
  • 　　http://www.cplusplus.com/reference/cctype/isalnum/?kw=isalnum
• transform - C++ Reference
  • 　　http://www.cplusplus.com/reference/algorithm/transform/?kw=transform
• string::push_back - C++ Reference
  • 　　http://www.cplusplus.com/reference/string/string/push_back/
• reverse_copy - C++ Reference
  • 　　http://www.cplusplus.com/reference/algorithm/reverse_copy/
• reverse_copy
  • 　　https://msdn.microsoft.com/zh-cn/library/c7ty5c36.aspx

 1 #include <iostream>
 2 #include <string>
 3 #include <cctype>  // toupper, isalnum
 4 #include <algorithm>  // transform
 5 using namespace std;
 6 
 7 class Solution {
 8 public:
 9     bool isPalindrome(string s) 
10     {
11          string sTemp = "", sReverse = "";
12 
13          // Convert characters to UPPER
14          transform(s.begin(), s.end(), s.begin(), ::toupper);
15 
16          // considering only alphanumeric characters
17          for (int i = 0; i < s.length(); i ++)
18              if (isalnum(s.at(i)))
19                 sTemp.push_back(s.at(i)); // Appends character to the end of the string, increasing its length by one.
20 
21          // Need to allocate space if use reverse_copy
22          sReverse.resize(sTemp.length());
23          reverse_copy(sTemp.begin(), sTemp.end(), sReverse.begin());
24 /*
25          sReverse = sTemp;
26          reverse(sReverse.begin(), sReverse.end());
27 */         
28 /*
29          cout << sTemp << endl;
30          cout << sReverse << endl;
31 */         
32          return sTemp == sReverse;
33     }
34     
35     bool isPalindrome2(string s) 
36     {
37          int start = 0, end = s.length() - 1;
38 
39          while (start < end)
40          {
41                if ( !isalnum(s.at(start)) ) 
42                   start ++;
43                else if ( !isalnum(s.at(end)) ) 
44                     end --;
45                else if (toupper(s.at(start)) != toupper(s.at(end))) // Remember toupper 
46                       return false;
47                else
48                {
49                    // Remember to move ptr
50                    start ++;
51                    end --;
52                }
53          }
54          
55          return true;
56     }
57     
58 };
59 
60 int main ()
61 {
62     Solution testSolution;
63     string sTest[] = {"A man, a plan, a canal: Panama", "race a car"};
64     
65     for (int i = 0; i < 2; i ++)
66         cout << testSolution.isPalindrome(sTest[i]) << endl;
67     
68     getchar();
69     
70     return 0;
71 }

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