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11. Same Tree

easyAsked at Coursera

Decide if two binary trees are structurally identical — Coursera tests recursive structural compare for course-tree diffing.

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 have the same node values.

Constraints

  • 0 <= nodes <= 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

Preorder serialize both with null markers, 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. Recursion

Compare both nulls, then values, then recurse children.

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:

Coursera-specific tips

Coursera asks you to handle the null-vs-null base case first — they have seen candidates flunk by forgetting the (!p && !q) case in their course-tree compare ETL.

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Output

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