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11. Same Tree

easyAsked at GitHub

Determine if two binary trees are identical — GitHub's stand-in for SHA-equality checks across two commit subtrees in a fast-forward verification.

By Alex Chen, Founder, InterviewChamp.AI · Last verified

Problem

Given the roots of two binary trees p and q, return true if they are structurally identical and corresponding nodes have the same value.

Constraints

  • Nodes in [0, 100]
  • -10^4 <= Node.val <= 10^4

Examples

Example 1

Input
p = [1,2,3], q = [1,2,3]
Output
true

Example 2

Input
p = [1,2], q = [1,null,2]
Output
false

Approaches

1. Serialize + compare

Serialize each tree with null markers, string-equal them.

Time
O(n)
Space
O(n)
const ser = (n) => !n ? '#' : `${n.val},${ser(n.left)},${ser(n.right)}`;
return ser(p)===ser(q);

Tradeoff:

2. Parallel recursion

Recurse both trees in lockstep; short-circuit on first mismatch or structural divergence.

Time
O(n)
Space
O(h)
function isSameTree(p, q) {
  if (!p && !q) return true;
  if (!p || !q) return false;
  if (p.val !== q.val) return false;
  return isSameTree(p.left, q.left) && isSameTree(p.right, q.right);
}

Tradeoff:

GitHub-specific tips

GitHub probes whether you short-circuit on the first mismatch — same optimization used in tree-diff to skip subtrees that share root SHA.

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Output

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