Left View of Binary Tree (DSA)

 

Problem statement:(👈click here for GFG)

Given a Binary Tree, return Left view of it. Left view of a Binary Tree is set of nodes visible when tree is visited from Left side. The task is to complete the function leftView(), which accepts root of the tree as argument.

Left view of following tree is 1 2 4 8.

          1
       /     \
     2        3
   /     \    /    \
  4     5   6    7
   \
     8   

Example 1:

Input:
  1
 /  \
3    2
Output: 1 3

Example 2:

Input:

Output: 10 20 40

Solution:
class Solution {
    leftView(root) {
        let result = [];
        let maxLevel = {val: 0};
        this.leftViewUtil(root, 1, maxLevel, result);
        return result;
    }

    leftViewUtil(node, level, maxLevel, result) {
        if (node == null) return;
        if (level > maxLevel.val) {
            result.push(node.data);
            maxLevel.val = level;
        }
        this.leftViewUtil(node.left, level + 1, maxLevel, result);
        this.leftViewUtil(node.right, level + 1, maxLevel, result);
    }
}

Solution:2:

// Definition of a binary tree node
class Node {
    constructor(data) {
        this.data = data;
        this.left = null;
        this.right = null;
    }
}

// Solution class to get the left view of a binary tree
class Solution {
    leftView(root) {
        let result = [];
        let maxLevel = {val: 0};
        this.leftViewUtil(root, 1, maxLevel, result);
        return result;
    }

    leftViewUtil(node, level, maxLevel, result) {
        if (node == null) return;
        if (level > maxLevel.val) {
            result.push(node.data);
            maxLevel.val = level;
        }
        this.leftViewUtil(node.left, level + 1, maxLevel, result);
        this.leftViewUtil(node.right, level + 1, maxLevel, result);
    }
}

// Helper function to build a binary tree from an array
function buildTree(arr) {
    // Helper function to build the binary tree recursively
    function buildTreeHelper(index) {
        // Base case: index out of range or null value in array
        if (index >= arr.length || arr[index] == null) {
            return null;
        }
        // Create the current node
        const node = new Node(arr[index]);
        // Recursively build the left and right subtrees
        node.left = buildTreeHelper(2 * index + 1);
        node.right = buildTreeHelper(2 * index + 2);
        return node;
    }
    // Start building the binary tree from the root node
    return buildTreeHelper(0);
}

// Example usage
const arr = [1, 2, 3, null, 4, 5, 6, null, null, 7, 8];
const root = buildTree(arr);
const solution = new Solution();
console.log(solution.leftView(root)); // output: [1, 2, 4, 7]