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11. Same Tree

easyAsked at Udemy

Decide whether two binary trees are structurally and value-equal — Udemy uses this to test recursive equality before harder course-tree diff problems.

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

Problem

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

Constraints

  • 0 <= nodes in each tree <= 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

Serialize both trees with explicit nulls, then compare strings.

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

Tradeoff:

2. Recurse pairs

If both null, equal. If only one null, unequal. Else compare values and recurse on left and right pairs.

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

Tradeoff:

Udemy-specific tips

Udemy interviewers like to extend this to diffing two versions of a course outline — show you handle null branches before touching values.

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Output

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