LeetCode Day3

 Lowest Common Ancestor of a Binary Search Tree

 import java.util.ArrayList;
 import java.util.List;

 /**
  * LeetCode: Lowest Common Ancestor of a Binary Search Tree
  * Given a binary search tree (BST), find the lowest common ancestor (LCA) of two given nodes in the BST
  *
  * @author LuoPeng
  * @time 215.8.5
  *
  */
 public class LowestCommonAncestor {

     /**
      * If a node A is the common ancestor, and its left child and right child are not at the same time.
      * A is the Lowest Common Ancestor
      *
      * @param root the root of the tree
      * @param p
      * @param q
      * @return lowest common ancestor (LCA) of p and q
      */
     public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {

         if ( root == null || p == root || q == root) {return root;}

         TreeNode lca = null;
         /*
          * If one child is null, the lowest common ancestor must be the child of the other child of root
          */
         if ( root.left == null) {
             lca = lowestCommonAncestor(root.right, p, q);
         } else if ( root.right == null) {
             lca = lowestCommonAncestor(root.left, p, q);
         } else {
             boolean first = isCommonAncestor(root.left, p, q);
             boolean second = isCommonAncestor(root.right, p, q);
             if ( first) {
                 // if root.left is a common ancestor, the LCA must be root.left or a child of it.
                 lca = lowestCommonAncestor(root.left, p, q);
             } else if (second) {
                 // if root.right is a common ancestor, the LCA must be root.right or a child of it.
                 lca = lowestCommonAncestor(root.right, p, q);
             } else {
                 // For root is a common ancestor of p and q, the LCA must be root if the left child
                 // and right child are not the common ancestors.
                 lca = root;
             }
         }

         return lca;
     }

     /**
      * Whether root is the common ancestor of p and q
      *
      * @param root a node
      * @param p a node
      * @param q a node
      * @return True if root is the common ancestor of p and q, otherwise false.
      */
     private boolean isCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
         if ( root == null) {return false;}

         TempQueue queue = new TempQueue();
         TreeNode temp = null;
         boolean first = false;
         boolean second = false;

         // Breadth First Search
         queue.push(root);
         while ( !queue.empty()) {
             temp = queue.peek();
             queue.pop();
             if ( temp ==p) {
                 first = true;
             } else if ( temp == q) {
                 second = true;
             }
             // add the child
             if ( temp.left != null) {
                 queue.push(temp.left);
             }
             if ( temp.right != null) {
                 queue.push(temp.right);
             }
             // break if p and q have bean found
             if ( first && second) {
                 break;
             }
         }

         return first && second;
     }

 }

 /**
  * Queue
  *
  */
 class TempQueue {
     public void push(TreeNode x) {
         values.add(x);
     }

     public void pop() {
         values.remove(0);
     }

     public TreeNode peek() {
         return values.get(0);
     }

     public int size() {
         return values.size();
     }

     public boolean empty() {
         return values.size()==0;
     }

     private List<TreeNode> values = new ArrayList<TreeNode>();
 }