k1xme

/*
* Space complexity: O(1); Time complexity: O(n).
*/
void reverseString(char* str){
  if(str == NULL) return; // Validation checking.
  if(*str == "\0") return; // Empty string.
  
  char temp; // For swapping tail and head.
  
  char* head=str, tail=str; // Pointers for the beginning and the end of the string.

k1xme

class Solution{
  public static boolean isPermutation(String s1, String s2){
    if(s1 === null || s2 == null ) return false;
    if(s1.length() != s2.length()) return false;
    if(s1.length() == 0) return true; // If s1.length() == s2.length() and s1 is a empty string, s2 is the permutation of s1.
    
    char[] sc1 = s1.toCharArray();
    char[] sc2 = s2.toCharArray();
    
    HashMap<String, int> temp = new HashMap<String, boolean>();

k1xme

class Solution{
  // Space complexity: O(n);
  // Time complexity: O(n);
  public static char[] replaceSpaces(char[] str){
    if(str == null) return null;
    if(str.length == 0 ) return str;
    
    int length = 3 * str.length; // Triple its original size to ensure enough space to hold the result.
    int i = 0; // The index of current character.
    char[] result = new char[length]; // Create a new array to store the result string.

k1xme

class Solution{
    /*
    * Space complexity: O(n); Time complexity: O(n).
    */
	  
	  public static String compression(String str){
	    if(str == null || str.isEmpty()) return str;
	    
	    char[] chars = str.toCharArray();
	    StringBuffer result = new StringBuffer();

k1xme

class Solution{
  
  public static int[][] rotate(int[][] image){
    int n = image[0].length;
    int layer = 0;
    int temp;
    
    for(;layer< n/2; layer++){
      for(int i = layer; i< n - layer - 1; i++){
        temp = image[layer][i];

k1xme

class Solution{
	  /*
	  * Space: O(1); Time: O(n^2).
	  */
	  public static int[][] rotate(int[][] image){
	    if(image == null) return image;
	    
	    int n = image[0].length;
	    
	    if(n != image.length) return image;

k1xme

class Solution{
	/*
	* Space: O(n); Time(n^2)
	*/
	public static void setZero(int[][] matrix){
		if(matrix == null) return;
		
		int row_num = martix.length;
		int col_num = martix[0].length;