88.Lowest Common Ancestor

1.Description(Medium)

Given the root and two nodes in a Binary Tree. Find the lowest common ancestor(LCA) of the two nodes.

The lowest common ancestor is the node with largest depth which is the ancestor of both nodes.

Notice

Assume two nodes are exist in tree.

Example

For the following binary tree:

  4
 / \
3   7
   / \
  5   6

LCA(3, 5) =4

LCA(5, 6) =7

LCA(6, 7) =7

2.Code

public TreeNode lowestCommonAncestor(TreeNode root, TreeNode A, TreeNode B) {
        if(root==null || root==A || root==B){//当前节点为空或者是AB其中一个就返回当前节点
            return root;
        }

        //Divided
        TreeNode left=lowestCommonAncestor(root.left,A,B);
        TreeNode right=lowestCommonAncestor(root.right,A,B);

        //Conquer
        if(left!=null && right!=null){  //AB位于左右子树两侧
            return root;
        }
        if(left!=null){            //此处右子树应该为空，因为找不到A,B任何一个，AB全在左子树上
            return left;
        }
        if(right!=null){
            return right;
        }

        return null;