LeetCode – Lowest Common Ancestor of a Binary Tree (Java)

Given a binary tree, find the lowest common ancestor (LCA) of two given nodes in the tree.

Java Solution 1

public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
    if(root==null)
        return null;
 
    if(root==p || root==q)
        return root;
 
    TreeNode l = lowestCommonAncestor(root.left, p, q);
    TreeNode r = lowestCommonAncestor(root.right, p, q);
 
    if(l!=null&&r!=null){
        return root;
    }else if(l==null&&r==null){
        return null;
    }else{
        return l==null?r:l;
    }
}

To calculate time complexity, we know that f(n)=2*f(n-1)=2*2*f(n-2)=2^(logn), so time=O(n).

Java Solution 2 - deprecated

Please use the following diagram to walk through the solution.

Lowest Common Ancestor of a Binary Tree

Since each node is visited in the worst case, time complexity is O(n).

class Entity{
    public int count;
    public TreeNode node;
 
    public Entity(int count, TreeNode node){
        this.count = count;
        this.node = node;
    }
}
 
public class Solution {
    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
        return lcaHelper(root, p, q).node;
    }
 
    public Entity lcaHelper(TreeNode root, TreeNode p, TreeNode q){
        if(root == null) return new Entity(0, null);
 
        Entity left = lcaHelper(root.left, p, q);
        if(left.count==2) 
            return left;
 
        Entity right = lcaHelper(root.right,p,q);
        if(right.count==2) 
            return right;
 
        int numTotal = left.count + right.count;
        if(root== p || root == q){
            numTotal++;
        }
 
        return new Entity(numTotal, root);      
    }
}

Category >> Algorithms

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Kishore Srinivas

Solution 2 is the correct one, as the first solution will fail if one of the node is not in the tree, I wonder why is that marked as deprecated ?
Rajesh

I created my solution : LCA of BST
Cesar Gonzalez

your solution is great, but I think that applies only if is a BST, not for a simple Binary Tree.
http://www.programcreek.com/2014/07/leetcode-lowest-common-ancestor-of-a-binary-search-tree-java/
Birendra Singh

Here is my recursive version of the solution-
static TreeNode lowestCommAncesterRecu( TreeNode root, int a, int b)
{
if(root != null )
{
if(root.value == a || root.value == b){
return root;
}
if(root.value > a && root.value < b)
{
return root;
}

if(root.value b)
{
return root;
}

if(root.value > a && root.value > b)
{
root = lowestCommAncesterRecu(root.leftNode,a,b);
}

if(root.value < a && root.value < b)
{
root =lowestCommAncesterRecu(root.rightNode,a,b);
}
}
return root;
}
Titan

Following code working tested :

public int lca(TreeNode a, int val1, int val2) {
        if(find(a, val1) == -1 || find(a, val2) == -1) return -1;
        TreeNode l = LCA(a,val1,val2);
        return l == null ? -1 : l.val;
}

public TreeNode LCA(TreeNode a, int val1, int val2) {
        if(a == null) return null;
        if(a.val == val1 || a.val == val2) return a;
        TreeNode l1 = LCA(a.left, val1, val2);
        TreeNode l2 = LCA(a.right, val1, val2);
        if(l1 != null && l2 != null) return a;
        if(l1 == null && l2 == null) return null;
        return l1 != null ? l1 : l2;
}

public int find(TreeNode a, int val){
        if(a == null) return -1;
        if(a.val == val) return 1;
        return Math.max(find(a.left, val), find(a.right, val));
}
nidhi pitroda

Solution 1 doesn’t work for the case when one of the nodes in ancestor of the other. Correct Solution should be as follows :

public class Solution { Boolean flag1,flag2; public int lca(TreeNode a, int val1, int val2) { flag1 = flag2 = false; if(a==null) return -1; if(a.left == null && a.right == null) { return (a.val == val1 && val1==val2) ? a.val : -1; } TreeNode temp = find_lca(a,val1,val2); if(flag1 && flag2) return temp.val; else return -1; } public TreeNode find_lca(TreeNode node,int val1,int val2) { TreeNode lca=null; if(node == null) return null; if(node.val == val1) { this.flag1 = true; lca = node; //return node; } if(node.val == val2) { this.flag2 = true; lca = node; // return node; } TreeNode l = find_lca(node.left,val1,val2); TreeNode r = find_lca(node.right,val1,val2); if((l != null && r!=null) || (lca != null)) return node; return l!=null ? l:r; } }
Hooman

Here’s another solution (java):
https://gist.github.com/hmny/cbfc98029ae746dbd14a83f96c294ef7
Cherry Zhao

This is a great analysis. I also read an in-depth discussion about this question here http://bit.ly/29kAkXM.
Ameya Naik

Good catch.. Alternatively, we could have:
numTotal += (root == p ? 1 : 0) + (root == q ? 1 : 0);
w4nderlust

Solution 1 doesn’t work in the case of p == q. Should replace

if(root== p || root == q){
numTotal++;
}

with

if (root == p) {
numTotal ++;
}
if (root == q) {
numTotal ++;
}

or add a check at the beginning.
ryanlr

Tried on leetcode online judge, and it didn’t pass.
public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) { return findLCA(root, p.val, q.val); }
Ankit Shah

The above code doesn’t seem to work if Btree happens to be BST. Also here is the more simpler one.

public class LcaBinaryTree { TreeNode findLCA(TreeNode root, int n1, int n2) { // Base case if (root == null) return null; // If either n1 or n2 matches with root's key, report // the presence by returning root (Note that if a key is // ancestor of other, then the ancestor key becomes LCA if (root.data == n1 || root.data == n2) return root; // Look for keys in left and right subtrees TreeNode left_lca = findLCA(root.left, n1, n2); TreeNode right_lca = findLCA(root.right, n1, n2); // If both of the above calls return Non-NULL, then one key // is present in once subtree and other is present in other, // So this node is the LCA if (left_lca != null && right_lca != null) return root; // Otherwise check if left subtree or right subtree is LCA return (left_lca != null) ? left_lca : right_lca; } public static void main(String[] args) { TreeNode root = new TreeNode(1); root.left = new TreeNode(2); root.right = new TreeNode(3); root.left.left = new TreeNode(4); root.left.right = new TreeNode(5); root.right.left = new TreeNode(6); root.right.right = new TreeNode(7); // BTreePrinter.printNode(root); LcaBinaryTree lca = new LcaBinaryTree(); System.out.println("lca(6 , 7): " + lca.findLCA(root, 6, 7).data); System.out.println("lca(4 , 5): " + lca.findLCA(root, 4, 5).data); System.out.println("lca(2 , 3): " + lca.findLCA(root, 2, 3).data); } }

Output of the program:
1 / / 2 3 / / 4 5 6 7
lca(6 , 7): 3 lca(4 , 5): 2 lca(2 , 3): 1
Sirius

Could you please explain the code and its complexity? Thanks!

LeetCode – Lowest Common Ancestor of a Binary Tree (Java)

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