LCA III

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. Returnnullif LCA does not exist.

Example

For the following binary tree:

  4
 / \
3   7
   / \
  5   6

LCA(3, 5) =4

LCA(5, 6) =7

LCA(6, 7) =7

node A or node B may not exist in tree.

Note

需要ResultType来记录a存不存在，b存不存在。

boolean a_exist = left_rt.a_exist || right_rt.a_exist || A == root;

    boolean b\_exist = left\_rt.b\_exist \|\| right\_rt.b\_exist \|\| B == root;

结果要求左右都存在

Code

/**
 * Definition of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 */


public class Solution {
    class ResultType {
        public boolean a_exist, b_exist;
        public TreeNode node;
        public ResultType(boolean a_exist, boolean b_exist, TreeNode node) {
            this.a_exist = a_exist;
            this.b_exist = b_exist;
            this.node = node;
        }
    }
    /*
     * @param root: The root of the binary tree.
     * @param A: A TreeNode
     * @param B: A TreeNode
     * @return: Return the LCA of the two nodes.
     */
    public TreeNode lowestCommonAncestor3(TreeNode root, TreeNode A, TreeNode B) {
        // write your code here
        ResultType res = helper(root, A, B);

        if (res.a_exist && res.b_exist) {
            return res.node;
        } else {
            return null;
        }
    }

    private ResultType helper(TreeNode root, TreeNode A, TreeNode B) {
        if (root == null) {
            return new ResultType(false, false, null);
        }

        ResultType left_rt = helper(root.left, A, B);
        ResultType right_rt = helper(root.right, A, B);

        boolean a_exist = left_rt.a_exist || right_rt.a_exist || A == root;
        boolean b_exist = left_rt.b_exist || right_rt.b_exist || B == root;

        if (root == A || root == B) {
            return new ResultType(a_exist, b_exist, root);
        }

        if (left_rt.node != null && right_rt.node != null) {
            return new ResultType(a_exist, b_exist, root);
        }

        if (left_rt.node != null) {
            return new ResultType(a_exist, b_exist, left_rt.node);
        }

        if (right_rt.node != null) {
            return new ResultType(a_exist, b_exist, right_rt.node);
        }

        return new ResultType(a_exist, b_exist, null);
    }
}