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7. Same Tree

easyAsked at LINE

Decide if two binary trees are structurally identical and have equal values at every node — LINE uses this to test recursion clarity before moving into chat-tree replication design.

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 considered 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. Brute force serialization

Serialize both trees to strings with null markers, then 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 pairwise

Walk both trees in lockstep; at each step compare values and recurse on left and right pairs. Short-circuit on the 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:

LINE-specific tips

At LINE, frame this as the diff check you'd run when reconciling a chat-history tree between two devices after offline edits — concrete fan-out language helps.

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Output

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