Leetcode 236. Lowest Common Ancestor of a Binary Tree

Clarification

Brute-force Approach

Optimization

Code

class Solution {
    TreeNode lca = null;

    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
        seen(root, p, q);

        return lca;
    }

    private int seen(TreeNode root, TreeNode p, TreeNode q) {
        if (root == null || lca != null)   return 0;

        int left = seen(root.left, p, q);
        int right = seen(root.right, p, q);
        int seen = left + right +
                    (root == p ? 1 : 0) +
                    (root == q ? 1 : 0);
        if (lca == null && seen == 2)    lca = root;

        return seen;
    }
}

Follow-up