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10. Same Tree

easyAsked at Ola

Determine whether two binary trees are structurally identical with equal values.

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 value.

Constraints

  • Number of nodes in each tree is 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 and compare

Serialize both with sentinels for nulls and compare strings.

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. DFS in parallel

Walk both trees together; bail on the first mismatch or null gap. O(min(n)) and O(h) stack.

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:

Ola-specific tips

Ola asks this to test parallel recursion fluency; mention the practical use of comparing two snapshots of a city-zone tree without rebuilding it.

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