Lowest Common Ancestor of a Binary Tree

Given a binary tree, find the lowest common ancestor (LCA) of two given nodes 

in the tree.

        _______3______
       /              \
    ___5__          ___1__
   /      \        /      \
   6      _2       0       8
         /  \
         7   4

For example, the lowest common ancestor (LCA) of nodes 5 and 1 is 3. Another 

example is LCA of nodes 5 and 4 is 5, since a node can be a descendant of itself 

according to the LCA definition.

也可以用递归来实现。

在root为根的二叉树中找A,B的LCA:

如果找到了就返回这个LCA

如果只碰到A，就返回A

如果只碰到B，就返回B

如果都没有，就返回null

public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
	if(root==null||root==p||root==q)
		return root;
	TreeNode left=lowestCommonAncestor(root.left, p, q);
	TreeNode right=lowestCommonAncestor(root.right, p, q);
	if(left!=null&&right!=null)
		return root;
	if(left!=null)
		return left;
	if(right!=null)
		return right;
	return null;
}