344. Reverse String【easy】

344. Reverse String【easy】

Write a function that takes a string as input and returns the string reversed.

Example:
Given s = "hello", return "olleh".

解法一：

 class Solution {
 public:
     string reverseString(string s) {
         int start = , end = s.length() - ;

         while (start <= end) {
             char c = s[start];
             s[start] = s[end];
             s[end] = c;
             ++start;
             --end;
         }

         return s;
     }
 };

解法二：

 class Solution {
 public:
     void reverseStringHelper(string & s, int len, int start, string & result)
     {
         if (start == len) {
             return;
         }

         reverseStringHelper(s, len, start + , result);

         result += s[start];
     }

     string reverseString(string s) {
         string result;

         reverseStringHelper(s, s.length(), , result);

         return result;
     }
 };

递归

解法三：

 public class Solution {
     public String reverseString(String s) {
         int length = s.length();
         if (length <= ) return s;
         String leftStr = s.substring(, length / );
         String rightStr = s.substring(length / , length);
         return reverseString(rightStr) + reverseString(leftStr);
     }
 }

参考@ratchapong.t 的代码

Complexity Analysis

Time Complexity: `O(n log(n))` (Average Case) and `O(n * log(n))` (Worst Case) where `n` is the total number character in the input string. The recurrence equation is `T(n) = 2 * T(n/2) + O(n)`. `O(n)` is due to the fact that concatenation function takes linear time. The recurrence equation can be solved to get `O(n * log(n))`.

Auxiliary Space: `O(h)` space is used where `h` is the depth of recursion tree generated which is `log(n)`. Space is needed for activation stack during recursion calls.

Algorithm

Approach: Divide and Conquer (Recursive)

The string is split into half. Each substring will be further divided. This process continues until the string can no longer be divided (length `<= 1`). The conquering process will take they previously split strings and concatenate them in reverse order.