Lowest Common Ancestor of a Binary Tree

mediumcodingAlgorithmsBinary TreesDFS~25 min

Asked at Google, Meta

Overview

Given a binary tree and two nodes p and q, find their lowest common ancestor (LCA) — the deepest node that has both p and q as descendants (a node can be a descendant of itself).

Constraints

The number of nodes is in the range [2, 100,000].
All node values are unique.
p !== q and both exist in the tree.

Examples

// Tree: [3,5,1,6,2,0,8,null,null,7,4]
lowestCommonAncestor(root, 5, 1); // => 3
lowestCommonAncestor(root, 5, 4); // => 5

Solution

Reveal solution

function lowestCommonAncestor(root, p, q) {
  if (!root || root.val === p || root.val === q) return root;
  const left = lowestCommonAncestor(root.left, p, q);
  const right = lowestCommonAncestor(root.right, p, q);
  if (left && right) return root;
  return left || right;
}

Resources

Lowest Common Ancestor — LeetCode

lowest-common-ancestor-binary-tree.js

Lowest Common Ancestor of a Binary Tree

mediumcodingAlgorithmsBinary Trees

Overview

Given a binary tree and two nodes p and q, find their lowest common ancestor (LCA) — the deepest node that has both p and q as descendants (a node can be a descendant of itself).

Constraints

The number of nodes is in the range [2, 100,000].
All node values are unique.
p !== q and both exist in the tree.

Examples

// Tree: [3,5,1,6,2,0,8,null,null,7,4]
lowestCommonAncestor(root, 5, 1); // => 3
lowestCommonAncestor(root, 5, 4); // => 5

Solution

Reveal solution

function lowestCommonAncestor(root, p, q) {
  if (!root || root.val === p || root.val === q) return root;
  const left = lowestCommonAncestor(root.left, p, q);
  const right = lowestCommonAncestor(root.right, p, q);
  if (left && right) return root;
  return left || right;
}

Resources

Lowest Common Ancestor — LeetCode

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