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10. Same Tree

easyAsked at Glassdoor

Determine whether two binary trees are identical in shape and values — Glassdoor uses this to grade recursive base-case discipline.

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

Problem

Given the roots of two binary trees p and q, write a function to check if they are the same tree. Two trees are the same if they are structurally identical and the nodes have the same values.

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

Stringify both trees with delimiters and compare.

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

Tradeoff:

2. Recursive DFS comparison

Both roots null = true; one null = false; otherwise compare values and recurse.

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:

Glassdoor-specific tips

Glassdoor grades for explicit null base cases — they value engineers who walk through them aloud since their review-deduplication trees hit deep-null paths constantly.

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Output

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