바이너리 트리의 왼쪽 보기

Problem Statement

Given the root of a binary tree, return the nodes visible when the tree is viewed from the left side.

Only the first node at each level is visible.

Brute Force Intuition

In an interview, you can explain it like this:

Perform a level-order traversal (BFS). For every level, simply record the first node encountered.

Since BFS already processes nodes level by level, identifying the first node is straightforward.

Complexity

• Time Complexity: O(N)
• Space Complexity: O(N)

Moving Towards the Optimal Approach

Notice that during BFS:

Each Level

↓

Processed Together

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If we know:

Level Size

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then the very first node processed at that level is exactly the left view.

Pattern Recognition

Whenever you see:

• Left View
• Right View
• Level Wise Processing

Think:

Level Order Traversal (BFS)

Key Observation

For every level:

First Node

↓

Left View

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Continue BFS until all levels are processed.

Optimal Approach

Step 1

Push the root into a queue.

Step 2

Process one level at a time.

For every level:

size = queue.size();

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Step 3

The first node:

i == 0

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belongs to the left view.

Step 4

Push:

Left Child

↓

Right Child

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into the queue.

Optimal Java Solution

class Solution {

    ArrayList<Integer> leftView(Node root) {

        ArrayList<Integer> ans = new ArrayList<>();

        if (root == null)
            return ans;

        Queue<Node> q = new LinkedList<>();

        q.offer(root);

        while (!q.isEmpty()) {

            int size = q.size();

            for (int i = 0; i < size; i++) {

                Node curr = q.poll();

                if (i == 0)
                    ans.add(curr.data);

                if (curr.left != null)
                    q.offer(curr.left);

                if (curr.right != null)
                    q.offer(curr.right);
            }
        }

        return ans;
    }
}

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Dry Run

         1
       /   \
      2     3
     / \     \
    4   5     6

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Level 1

1

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First Node:

1

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Level 2

2 3

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First Node:

2

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Level 3

4 5 6

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First Node:

4

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Final Answer:

[1,2,4]

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Why BFS Works?

BFS processes the tree level by level.

Since nodes are visited from left to right,

the first node encountered at every level is exactly the node visible from the left side.

Complexity Analysis

Metric Complexity Time Complexity O(N) Space Complexity O(N)

Interview One-Liner

Perform a level-order traversal and record the first node encountered at every level.

Pattern Learned

Level Order

↓

First Node

↓

Left View

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Similar Problems

• Left View of Binary Tree
• Right View of Binary Tree
• Top View
• Bottom View
• Zigzag Level Order Traversal

Memory Trick

Think:

One Level

↓

First Node

↓

Answer

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Mental Model

Queue

↓

Process One Level

↓

Pick First Node

↓

Continue

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Whenever you hear:

“Left View of Binary Tree”

your brain should immediately think:

Level Order Traversal + First Node of Every Level