236. Lowest Common Ancestor of a Binary Tree

Recursion

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) {
            return right;
        } else if (right == null) {
            return left;
        } else {
            return root;
        }
    }
}

Iterate

class Solution {
    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
        Map<TreeNode, TreeNode> childToParentMap = new IdentityHashMap<>();
        Set<TreeNode> parentSet = new HashSet<>();

        dfs(root, null, childToParentMap);

        while (p != null) {
            parentSet.add(p);
            p = childToParentMap.get(p);
        }

        while (q != null) {
            if (parentSet.contains(q)) {
                return q;
            }
            q = childToParentMap.get(q);
        }

        return null;
    }

    private void dfs(TreeNode node, TreeNode parent, Map<TreeNode, TreeNode> childToParentMap) {
        if (node == null) {
            return;
        }

        childToParentMap.put(node, parent);
        dfs(node.left, node, childToParentMap);
        dfs(node.right, node, childToParentMap);
    }
}

Reference

236. Lowest Common Ancestor of a Binary Tree
剑指 Offer 68 - II. 二叉树的最近公共祖先