樹的最近公共祖先問題

• 二叉搜尋樹的最近公共祖先
  題目連結：劍指 Offer 68 - I. 二叉搜尋樹的最近公共祖先
  235. 二叉搜尋樹的最近公共祖先

class Solution {
    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
        if (root == null || p == null || q == null) {
            return null;
        }

        if (Math.max(p.val, q.val) < root.val) {
            return lowestCommonAncestor(root.left, p, q);
        } else if (Math.min(p.val, q.val) > root.val) {
            return lowestCommonAncestor(root.right, p, q);
        } else {
            return root;
        }
    }
}

• 二叉樹的最近公共祖先
  連結：
  劍指 Offer 68 - II. 二叉樹的最近公共祖先
  236. 二叉樹的最近公共祖先
  面試題 04.08. 首個共同祖先
  

class Solution {
    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 null;
        } else if (left != null && right != null) {
            return root;
        } else if (left == null && right != null) {
            return right;
        } else {                 //left != null && right == null
            return left;
        }
    }
}