[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.

According to the definition of LCA on Wikipedia: “The lowest common ancestor is defined between two nodes v and w as the lowest node in T that has both v and w as descendants (where we allow a node to be a descendant of itself).”

        _______6______
       /              
    ___2__          ___8__
   /              /      
   0      _4       7       9
         /  
         3   5

For example, the lowest common ancestor (LCA) of nodes 2 and 8 is 6. Another example is LCA of nodes 2 and 4 is 2, since a node can be a descendant of itself according to the LCA definition.

做这道题，首先得了解二叉排序树的性质，然后就是利用队列去记录寻找到q或者p的路径，然后对路径进行遍历，找到分叉点，这个分叉点就是要找的点。

    public void findPath(TreeNode root, TreeNode p,Queue<TreeNode> queue)
    {
        TreeNode temp=root;
        while(temp!=null && temp.val!=p.val)
        {
            queue.offer(temp);
            if(temp.val>p.val) temp=temp.left;
            else temp=temp.right;
        }
        queue.add(p);
    }
    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
        Queue<TreeNode> pQ=new LinkedList<TreeNode>();
        Queue<TreeNode> qQ=new LinkedList<TreeNode>();
        findPath(root, p, pQ);
        findPath(root, q, qQ);
        TreeNode x1=null;
        TreeNode x2=null;
        TreeNode anc=null;
        while(!pQ.isEmpty() && !qQ.isEmpty())
        {
            x1=pQ.poll();
            x2=qQ.poll();
            if(x1.val==x2.val)
            {
                anc=x1;
            }
            else
            {
                break;
            }
        }
        return anc;
    }