Lowest Common Ancestor of a Binary Tree

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Example 1:

Input: root = [3,5,1,6,2,0,8,null,null,7,4], p = 5, q = 1
Output: 3
Explanation: The LCA of nodes 5 and 1 is 3.

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Example 2:

Input: root = [3,5,1,6,2,0,8,null,null,7,4], p = 5, q = 4
Output: 5
Explanation: The LCA of nodes 5 and 4 is 5, since a node can be a descendant of itself according to the LCA definition.

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Example 3:

Input: root = [1,2], p = 1, q = 2
Output: 1

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/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
        
        List<TreeNode> parentNodes1 = new ArrayList();
        List<TreeNode> parentNodes2 = new ArrayList();
        
        search(root, p, parentNodes1);
        search(root, q, parentNodes2);
        
        int n = Math.min(parentNodes1.size(), parentNodes2.size());
        TreeNode lca = root;
        for (int i = 1; i < n; i++) {
            if (parentNodes1.get(i).val == parentNodes2.get(i).val) {
                lca = parentNodes1.get(i);
            } else {
                break;
            }
        }
        return lca;

    }
    
    boolean search(TreeNode current, TreeNode target, List<TreeNode> path) {
        if (current == null) return false;
        
        path.add(current);
             
        if (current.val == target.val) {
            return true;
        }
        
        if (search(current.left, target, path)) {
            return true;
        }
        
        if (search(current.right, target, path)) {
            return true;
        }
        
        path.remove(path.size() - 1);
        
        return false;
    }
}

## Solution 2

```java
/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
     
       List<TreeNode> path1 = new ArrayList<>(); 
       search(root, p.val, path1);
        
       List<TreeNode> path2 = new ArrayList<>();
       search(root, q.val, path2);

       for (int i = Math.min(path1.size(), path2.size()) - 1; i >= 0; i--) {
            var p1 = path1.get(i);
            var p2 = path2.get(i);
            if (p1.val == p2.val) return p1;
        }
        return null;
    }
    
    TreeNode search(TreeNode node, int val, List<TreeNode> paths) {
        if (node == null) return null;
        
        paths.add(node);
        
        if (node.val == val) {
            return node;
        }
        
        TreeNode s = search(node.left, val, paths);
        if (s != null) return s;
        s = search(node.right, val, paths);
        if (s != null) return s;
        paths.remove(node);
        return null;
    }

}

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/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
        Stack<TreeNode> stack = new Stack<>();
         Map<TreeNode, TreeNode> nodeParent = new HashMap<>();
        
        stack.push(root);
        nodeParent.put(root, null);
        
        while (!nodeParent.containsKey(p) || !nodeParent.containsKey(q)) {
            TreeNode node = stack.pop();
            
            if (node.right != null) {
                nodeParent.put(node.right, node);
                stack.push(node.right);
            }
            
            if (node.left != null) {
                nodeParent.put(node.left, node);
                stack.push(node.left);
            }
        }
        
        Set<TreeNode> parents = new LinkedHashSet<>();
        while (p != null) {
            parents.add(p);
            p = nodeParent.get(p);
        }

        while (q != null && !parents.contains(q)) {
            q = nodeParent.get(q);
        }
        
        return q;
    }
}

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/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
        Stack<TreeNode> stack = new Stack<>();
        stack.push(root);
        
        Map<TreeNode, TreeNode> nodeParent = new HashMap<>();
        nodeParent.put(root, null);
        
        while (!nodeParent.containsKey(p) || !nodeParent.containsKey(q)) {
            TreeNode node = stack.pop();
            
            if (node.right != null) {
                nodeParent.put(node.right, node);
                stack.push(node.right);
            }
            
            if (node.left != null) {
                nodeParent.put(node.left, node);
                stack.push(node.left);
            }
        }
        
        
        Set<TreeNode> parents = new LinkedHashSet<>();
        // 5
        while (p != null) {
            parents.add(p);
            p = nodeParent.get(p);
        }
        
        while (q != null && !parents.contains(q)) {
            q = nodeParent.get(q);
        }
        
        return q;
    }
}