Added readme files.

Author: javadev

src/main/kotlin/g0001_0100/s0001_two_sum/readme.md

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+1\. Two Sum
+
+Easy
+
+Given an array of integers `nums` and an integer `target`, return _indices of the two numbers such that they add up to `target`_.
+
+You may assume that each input would have **_exactly_ one solution**, and you may not use the _same_ element twice.
+
+You can return the answer in any order.
+
+**Example 1:**
+
+**Input:** nums = [2,7,11,15], target = 9
+
+**Output:** [0,1]
+
+**Output:** Because nums[0] + nums[1] == 9, we return [0, 1]. 
+
+**Example 2:**
+
+**Input:** nums = [3,2,4], target = 6
+
+**Output:** [1,2] 
+
+**Example 3:**
+
+**Input:** nums = [3,3], target = 6
+
+**Output:** [0,1] 
+
+**Constraints:**
+
+*   <code>2 <= nums.length <= 10<sup>4</sup></code>
+*   <code>-10<sup>9</sup> <= nums[i] <= 10<sup>9</sup></code>
+*   <code>-10<sup>9</sup> <= target <= 10<sup>9</sup></code>
+*   **Only one valid answer exists.**
+
+**Follow-up:** Can you come up with an algorithm that is less than <code>O(n<sup>2</sup>) </code>time complexity?

src/main/kotlin/g0001_0100/s0002_add_two_numbers/readme.md

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+2\. Add Two Numbers
+
+Medium
+
+You are given two **non-empty** linked lists representing two non-negative integers. The digits are stored in **reverse order**, and each of their nodes contains a single digit. Add the two numbers and return the sum as a linked list.
+
+You may assume the two numbers do not contain any leading zero, except the number 0 itself.
+
+**Example 1:**
+
+![](https://assets.leetcode.com/uploads/2020/10/02/addtwonumber1.jpg)
+
+**Input:** l1 = [2,4,3], l2 = [5,6,4]
+
+**Output:** [7,0,8]
+
+**Explanation:** 342 + 465 = 807. 
+
+**Example 2:**
+
+**Input:** l1 = [0], l2 = [0]
+
+**Output:** [0] 
+
+**Example 3:**
+
+**Input:** l1 = [9,9,9,9,9,9,9], l2 = [9,9,9,9]
+
+**Output:** [8,9,9,9,0,0,0,1] 
+
+**Constraints:**
+
+*   The number of nodes in each linked list is in the range `[1, 100]`.
+*   `0 <= Node.val <= 9`
+*   It is guaranteed that the list represents a number that does not have leading zeros.

src/main/kotlin/g0001_0100/s0003_longest_substring_without_repeating_characters/readme.md

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+3\. Longest Substring Without Repeating Characters
+
+Medium
+
+Given a string `s`, find the length of the **longest substring** without repeating characters.
+
+**Example 1:**
+
+**Input:** s = "abcabcbb"
+
+**Output:** 3
+
+**Explanation:** The answer is "abc", with the length of 3. 
+
+**Example 2:**
+
+**Input:** s = "bbbbb"
+
+**Output:** 1
+
+**Explanation:** The answer is "b", with the length of 1. 
+
+**Example 3:**
+
+**Input:** s = "pwwkew"
+
+**Output:** 3
+
+**Explanation:** The answer is "wke", with the length of 3. Notice that the answer must be a substring, "pwke" is a subsequence and not a substring. 
+
+**Example 4:**
+
+**Input:** s = ""
+
+**Output:** 0 
+
+**Constraints:**
+
+*   <code>0 <= s.length <= 5 * 10<sup>4</sup></code>
+*   `s` consists of English letters, digits, symbols and spaces.

src/main/kotlin/g0001_0100/s0004_median_of_two_sorted_arrays/readme.md

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+4\. Median of Two Sorted Arrays
+
+Hard
+
+Given two sorted arrays `nums1` and `nums2` of size `m` and `n` respectively, return **the median** of the two sorted arrays.
+
+The overall run time complexity should be `O(log (m+n))`.
+
+**Example 1:**
+
+**Input:** nums1 = [1,3], nums2 = [2]
+
+**Output:** 2.00000
+
+**Explanation:** merged array = [1,2,3] and median is 2. 
+
+**Example 2:**
+
+**Input:** nums1 = [1,2], nums2 = [3,4]
+
+**Output:** 2.50000
+
+**Explanation:** merged array = [1,2,3,4] and median is (2 + 3) / 2 = 2.5. 
+
+**Example 3:**
+
+**Input:** nums1 = [0,0], nums2 = [0,0]
+
+**Output:** 0.00000 
+
+**Example 4:**
+
+**Input:** nums1 = [], nums2 = [1]
+
+**Output:** 1.00000 
+
+**Example 5:**
+
+**Input:** nums1 = [2], nums2 = []
+
+**Output:** 2.00000 
+
+**Constraints:**
+
+*   `nums1.length == m`
+*   `nums2.length == n`
+*   `0 <= m <= 1000`
+*   `0 <= n <= 1000`
+*   `1 <= m + n <= 2000`
+*   <code>-10<sup>6</sup> <= nums1[i], nums2[i] <= 10<sup>6</sup></code>

src/main/kotlin/g0001_0100/s0005_longest_palindromic_substring/readme.md

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+5\. Longest Palindromic Substring
+
+Medium
+
+Given a string `s`, return _the longest palindromic substring_ in `s`.
+
+**Example 1:**
+
+**Input:** s = "babad"
+
+**Output:** "bab" **Note:** "aba" is also a valid answer. 
+
+**Example 2:**
+
+**Input:** s = "cbbd"
+
+**Output:** "bb" 
+
+**Example 3:**
+
+**Input:** s = "a"
+
+**Output:** "a" 
+
+**Example 4:**
+
+**Input:** s = "ac"
+
+**Output:** "a" 
+
+**Constraints:**
+
+*   `1 <= s.length <= 1000`
+*   `s` consist of only digits and English letters.

src/main/kotlin/g0001_0100/s0006_zigzag_conversion/readme.md

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+6\. Zigzag Conversion
+
+Medium
+
+The string `"PAYPALISHIRING"` is written in a zigzag pattern on a given number of rows like this: (you may want to display this pattern in a fixed font for better legibility)
+
+P A H N A P L S I I G Y I R 
+
+And then read line by line: `"PAHNAPLSIIGYIR"`
+
+Write the code that will take a string and make this conversion given a number of rows:
+
+string convert(string s, int numRows); 
+
+**Example 1:**
+
+**Input:** s = "PAYPALISHIRING", numRows = 3
+
+**Output:** "PAHNAPLSIIGYIR" 
+
+**Example 2:**
+
+**Input:** s = "PAYPALISHIRING", numRows = 4
+
+**Output:** "PINALSIGYAHRPI"
+
+**Explanation:** P I N A L S I G Y A H R P I 
+
+**Example 3:**
+
+**Input:** s = "A", numRows = 1
+
+**Output:** "A" 
+
+**Constraints:**
+
+*   `1 <= s.length <= 1000`
+*   `s` consists of English letters (lower-case and upper-case), `','` and `'.'`.
+*   `1 <= numRows <= 1000`

src/main/kotlin/g0001_0100/s0007_reverse_integer/readme.md

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+7\. Reverse Integer
+
+Medium
+
+Given a signed 32-bit integer `x`, return `x` _with its digits reversed_. If reversing `x` causes the value to go outside the signed 32-bit integer range <code>[-2<sup>31</sup>, 2<sup>31</sup> - 1]</code>, then return `0`.
+
+**Assume the environment does not allow you to store 64-bit integers (signed or unsigned).**
+
+**Example 1:**
+
+**Input:** x = 123
+
+**Output:** 321 
+
+**Example 2:**
+
+**Input:** x = -123
+
+**Output:** -321 
+
+**Example 3:**
+
+**Input:** x = 120
+
+**Output:** 21 
+
+**Example 4:**
+
+**Input:** x = 0
+
+**Output:** 0 
+
+**Constraints:**
+
+*   <code>-2<sup>31</sup> <= x <= 2<sup>31</sup> - 1</code>

src/main/kotlin/g0001_0100/s0008_string_to_integer_atoi/readme.md

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+8\. String to Integer (atoi)
+
+Medium
+
+Implement the `myAtoi(string s)` function, which converts a string to a 32-bit signed integer (similar to C/C++'s `atoi` function).
+
+The algorithm for `myAtoi(string s)` is as follows:
+
+1.  Read in and ignore any leading whitespace.
+2.  Check if the next character (if not already at the end of the string) is `'-'` or `'+'`. Read this character in if it is either. This determines if the final result is negative or positive respectively. Assume the result is positive if neither is present.
+3.  Read in next the characters until the next non-digit character or the end of the input is reached. The rest of the string is ignored.
+4.  Convert these digits into an integer (i.e. `"123" -> 123`, `"0032" -> 32`). If no digits were read, then the integer is `0`. Change the sign as necessary (from step 2).
+5.  If the integer is out of the 32-bit signed integer range <code>[-2<sup>31</sup>, 2<sup>31</sup> - 1]</code>, then clamp the integer so that it remains in the range. Specifically, integers less than <code>-2<sup>31</sup></code> should be clamped to <code>-2<sup>31</sup></code>, and integers greater than <code>2<sup>31</sup> - 1</code> should be clamped to <code>2<sup>31</sup> - 1</code>.
+6.  Return the integer as the final result.
+
+**Note:**
+
+*   Only the space character `' '` is considered a whitespace character.
+*   **Do not ignore** any characters other than the leading whitespace or the rest of the string after the digits.
+
+**Example 1:**
+
+**Input:** s = "42"
+
+**Output:** 42
+
+**Explanation:** The underlined characters are what is read in, the caret is the current reader position.
+
+    Step 1: "42" (no characters read because there is no leading whitespace)
+             ^ 
+    Step 2: "42" (no characters read because there is neither a '-' nor '+')
+             ^
+    Step 3: "42" ("42" is read in)
+               ^
+
+The parsed integer is 42. Since 42 is in the range [-2<sup>31</sup>, 2<sup>31</sup> - 1], the final result is 42. 
+
+**Example 2:**
+
+**Input:** s = " -42"
+
+**Output:** -42
+
+**Explanation:**
+
+    Step 1: " -42" (leading whitespace is read and ignored)
+              ^ 
+    Step 2: " -42" ('-' is read, so the result should be negative)
+               ^
+    Step 3: " -42" ("42" is read in)
+                 ^
+    The parsed integer is -42.
+
+Since -42 is in the range [-2<sup>31</sup>, 2<sup>31</sup> - 1], the final result is -42. 
+
+**Example 3:**
+
+**Input:** s = "4193 with words"
+
+**Output:** 4193
+
+**Explanation:**
+
+    Step 1: "4193 with words" (no characters read because there is no leading whitespace)
+             ^
+    Step 2: "4193 with words" (no characters read because there is neither a '-' nor '+')
+             ^
+    Step 3: "4193 with words" ("4193" is read in; reading stops because the next character is a non-digit)
+                 ^ 
+    The parsed integer is 4193.
+
+Since 4193 is in the range [-2<sup>31</sup>, 2<sup>31</sup> - 1], the final result is 4193. 
+
+**Example 4:**
+
+**Input:** s = "words and 987"
+
+**Output:** 0
+
+**Explanation:**
+
+    Step 1: "words and 987" (no characters read because there is no leading whitespace)
+             ^
+    Step 2: "words and 987" (no characters read because there is neither a '-' nor '+')
+             ^
+    Step 3: "words and 987" (reading stops immediately because there is a non-digit 'w')
+             ^
+    The parsed integer is 0 because no digits were read.
+
+Since 0 is in the range [-2<sup>31</sup>, 2<sup>31</sup> - 1], the final result is 0. 
+
+**Example 5:**
+
+**Input:** s = "-91283472332"
+
+**Output:** -2147483648
+
+**Explanation:**
+
+    Step 1: "-91283472332" (no characters read because there is no leading whitespace)
+            ^
+    Step 2: "-91283472332" ('-' is read, so the result should be negative)
+              ^
+    Step 3: "-91283472332" ("91283472332" is read in)
+                         ^
+    The parsed integer is -91283472332.
+
+Since -91283472332 is less than the lower bound of the range [-2<sup>31</sup>, 2<sup>31</sup> - 1], the final result is clamped to -2<sup>31</sup> = -2147483648.  
+
+**Constraints:**
+
+*   `0 <= s.length <= 200`
+*   `s` consists of English letters (lower-case and upper-case), digits (`0-9`), `' '`, `'+'`, `'-'`, and `'.'`.