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10. Same Tree

easyAsked at Instacart

Decide if two binary trees are structurally identical — Instacart probes recursion fluency before comparing inventory snapshots across stores.

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

Problem

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

Constraints

  • Both trees have 0 to 100 nodes
  • -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 and compare

Stringify both trees in pre-order with null markers and compare.

Time
O(n)
Space
O(n)
function ser(n) {
  if (!n) return '#';
  return n.val + ',' + ser(n.left) + ',' + ser(n.right);
}
return ser(p) === ser(q);

Tradeoff:

2. Parallel recursion

Recurse on both trees in lockstep; bail out on first mismatch.

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:

Instacart-specific tips

Instacart wants the early-exit form — they'll ask how you'd extend it to confirm two inventory snapshots match across delivery regions.

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Output

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