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Concept

Fundamentals

The recursion template: base case → recurse → combine
step 1 / 23
binary tree
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call stack ↓
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Concept
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1count(node):
2    push frame
3    if node is null: return 0          // 1. base case
4    L = count(node.left)               // 2. recurse
5    R = count(node.right)              // 2. recurse
6    return 1 + L + R                   // 3. combine
state
  • templatebase → recurse → combine

line 1Every tree-DFS is built from the SAME three parts: (1) a base case, (2) recurse into the children, (3) combine their results at the parent. We will count the nodes in this tree to make all three concrete.