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 = 0, end = s.length() - 1;
        
        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 + 1, result);
        
        result += s[start];
    }
    
    string reverseString(string s) {
        string result;
        
        reverseStringHelper(s, s.length(), 0, result);
        
        return result;
    }
};

递归

解法三：

public class Solution {
    public String reverseString(String s) {
        int length = s.length();
        if (length <= 1) return s;
        String leftStr = s.substring(0, length / 2);
        String rightStr = s.substring(length / 2, 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.