LintCode : Lowest Common Ancestor

Description:

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.

Example:

For the following binary tree:

  4
 / \
3   7
   / \
  5   6

LCA(3, 5) = 4

LCA(5, 6) = 7

LCA(6, 7) = 7

/**
 * 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 {
    /**
     * @param root: The root of the binary search tree.
     * @param A and B: two nodes in a Binary.
     * @return: Return the least common ancestor(LCA) of the two nodes.
     */
    private List<TreeNode> res = new ArrayList<TreeNode>();
    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode A, TreeNode B){
        // write your code here
        helper(root, A, B);
        if (res.size() == 0) {
            return null;
        }
        return res.get(res.size() - 1);
    }
    public boolean isChild(TreeNode root, TreeNode x) {
        if (root == null) {
            return false;
        }
        if (root == x) {
            return true;
        }
        if (isChild(root.left, x) == true) {
            return true;
        }
        if (isChild(root.right, x) == true) {
            return true;
        }
        return false;
    }
    public boolean isCommonAncestor(TreeNode root, TreeNode A, TreeNode B){
        if (isChild(root, A) && isChild(root, B)) {
            return true;
        }
        return false;
    }
    public void helper(TreeNode root, TreeNode A, TreeNode B) {
        if (root == null) {
            return;
        }
        if (isCommonAncestor(root, A, B)) {
            res.add(root);
        }
        helper(root.left, A, B);
        helper(root.right, A, B);
    }
}

Find all the common ancestor and return the last one!