shixiaoyu

public void sortColorsCountingSort(int[] A) {
    if (A == null || A.length == 0) return;

    int[] buckets = new int[3];

    for (int i = 0; i < A.length; i++) {
        buckets[A[i]] += 1;
    }

    int index = 0;

shixiaoyu

public int reverseBits(int n) {
    int res = 0;

    for (int i = 0; i < 32; i++) {
        int t = n & 1;
        n = n >> 1;
        res = res << 1;
        res = res | t;

    }

shixiaoyu

public TreeNode invertTree(TreeNode root) {
    return this.helper(root);
}

// post order
private TreeNode helper(TreeNode root) {
    if (root == null) {
        return root;
    }

shixiaoyu

public class LRUCacheGoodVersion {

    public class Node {
        public int val;
        public int key;
        public Node pre = null;
        public Node next = null;

        public Node(int k, int v) {
            this.key = k;

shixiaoyu

// I really don't like inheritance, i prefer composition, but the removeEldestEntry is a protected method, have to do this
public class LRULinkedHashMap extends LinkedHashMap<Integer, Integer> {
    private int maxSize = -1;

    // This is simply use a LinkedHashMap, sort of cheating
    public LRULinkedHashMap(int capacity) {
        super(16, 0.75f, true);  // the 16 hashtable size is not to be confused with the max cache size
        this.maxSize = capacity;
    }

shixiaoyu

public int minDistance(String word1, String word2) {
    if (word1 == null || word2 == null) return -1;

    int l1 = word1.length();
    int l2 = word2.length();

    if (l1 == 0 || l2 == 0) {
        return l1 == 0 ? l2 : l1;
    }

shixiaoyu

public int shortestDistance(String[] words, String word1, String word2) {
    if (words == null || words.length == 0 || word1 == null || word2 == null || word1.equals(word2)) {
        return -1;
    }
    int minDistance = words.length;

    int wordIndex1 = -1;
    int wordIndex2 = -1;

    for (int i = 0; i < words.length; i++) {

shixiaoyu

public int maxSideLength(int[][] mat, int threshold) {
    if (mat == null || mat.length == 0 || mat[0].length == 0) {
        return 0;
    }
    int row = mat.length;
    int col = mat[0].length;
    // easier to implement with the initial value on the boarder to be 0
    int[][] prefixSum = new int[row + 1][col + 1];  

    // the idea of using a prefix sum matrix should be a common technique, each cell contains the sum of

shixiaoyu

public List<Integer> majorityElement(int[] nums) {
    List<Integer> res = new ArrayList<>();

    if (nums == null || nums.length == 0) {
        return res;
    }

    int candidate1 = 1;
    int candidate2 = 1;
    int vote1 = 0;

shixiaoyu

public String convertSimulationOptimizedImplementation(String s, int nRows) {
    if (s == null || s.length() == 0 || nRows <= 0) {
        return "";
    }

    if (nRows == 1) {
        return s;
    }

    int size = s.length();