给定一个二叉树, 找到该树中两个指定节点的最近公共祖先。

 private TreeNode2 lca = null;
    public TreeNode2 lowestCommonAncestor(TreeNode2 root, TreeNode2 p, TreeNode2 q) {
        if (root == null){
            return null;
        }
        findNode(root,p,q);
        return lca;
    }

    //看从root出发能不能找到p和q
    private boolean findNode(TreeNode2 root, TreeNode2 p, TreeNode2 q) {
        if (root == null){
            return false;
        }
        int left = findNode(root.left,p,q)?1:0;
        int right = findNode(root.right,p,q)?1:0;
        int mid = (root == p || root == q)?1:0;
        if (left + right + mid == 2){
            lca = root;
        }
        return (left + right + mid)>0;
    }