[LeetCode] Algorithm I - Day 4 Two Pointers

Algorithm I - Day 4 Two Pointers

344. Reverse String

Write a function that reverses a string. The input string is given as an array of characters s.

You must do this by modifying the input array in-place with O(1) extra memory.

Example 1:

Input: s = ["h","e","l","l","o"]
Output: ["o","l","l","e","h"]

Example 2:

Input: s = ["H","a","n","n","a","h"]
Output: ["h","a","n","n","a","H"]

 

Constraints:

• 1 <= s.length <= 105
• s[i] is a printable ascii character.

class Solution {
    public void reverseString(char[] s) {
        for(int i=0; i<s.length/2; i++){
            char box = s[i];
            s[i] = s[s.length-1-i];
            s[s.length-1-i] = box;
        }
    }
}

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557. Reverse Words in a String III

Given a string s, reverse the order of characters in each word within a sentence while still preserving whitespace and initial word order.

 

Example 1:

Input: s = "Let's take LeetCode contest"
Output: "s'teL ekat edoCteeL tsetnoc"

Example 2:

Input: s = "God Ding"
Output: "doG gniD"

 

Constraints:

• 1 <= s.length <= 5 * 104
• s contains printable ASCII characters.
• s does not contain any leading or trailing spaces.
• There is at least one word in s.
• All the words in s are separated by a single space.

class Solution {
    // Approach:
    // Using two pointers, 'start' and 'end', each time 'end' reaches a space, we reverse the words before.
    // To reverse, we convert the String 's' to a char array, then write a method to reverse the char in the array.

    public String reverseWords(String s) {
        // Convert s into char array.
        char[] chars = s.toCharArray();

        int start = 0;
        int end;
        for (end = 0; end <= chars.length; end++) {
            // When 'end' reaches a space character ' ', reverse the words from 'start' to 'end' - 1 index in 'chars'.
            if (end == chars.length || chars[end] == ' ') {
                reverse(chars, start, end - 1);
                // reset the start for the next word.
                start = end + 1;
            }
        }

        return new String(chars);
    }

    // A private method to reverse the char in the array using the 'start' and 'end' pointer.
    private void reverse(char[] c, int start, int end) {
        while (start < end) {
            char tmp = c[end];
            c[end] = c[start];
            c[start] = tmp;
            start++;
            end--;
        }
    }
}