Exercises with binary trees in Javascript

binary_trees.js

class TreeNode {
    constructor(value){
        this.value = value;
        this.left = null;
        this.right = null;
    }

    /*
    "A class may have only a single constructor"
    constructor(value, left, right){
        this.value = value;
        this.left = left;
        this.right = right;
    }
    */
}

function depth_first_iterative_alg(root){
    if (root == null) return [];

    const stack = [root];
    const result = [];
    while (stack.length > 0){
        const curr_node = stack.pop();
        //console.log(curr_node.value);
        result.push(curr_node); // If I push curr_node.value, I will instead return the array of values instead of TreeNode
        if (curr_node.right) stack.push(curr_node.right);   // The "if (curr_node.right)" condition is equivalent
        if (curr_node.left) stack.push(curr_node.left);     // to "if (curr_node.right != null)"

    }

    return result;
}

function depth_first_recursive_alg(root){
    if (root == null) return [];

    const left_values = depth_first_recursive_alg(root.left);
    const right_values = depth_first_recursive_alg(root.right);

    return [root].concat(left_values, right_values);
    //The return could also be carried out via the Spread operator:
    //return [root, ...left_values, ...right_values];
}

// Breadth-first algorithm is easily implementable only in an iterative form
function breadth_first_alg(root){
    if (root == null) return [];

    const queue = [root];
    const result = [];
    while (queue.length > 0){
        const curr_node = queue.shift();
        //console.log(curr_node.value);
        result.push(curr_node);
        if (curr_node.left != null) queue.push(curr_node.left);
        if (curr_node.right != null) queue.push(curr_node.right);
    }

    return result;
}



// binary tree test example:
//
//          a
//        /   \
//       b     c
//     /  \     \
//   d      e     f
//
const a = new TreeNode("a");
const b = new TreeNode("b");
const c = new TreeNode("c");
const d = new TreeNode("d");
const e = new TreeNode("e");
const f = new TreeNode("f");
a.left = b;
a.right = c;
b.left = d;
b.right = e;
c.right = f;

console.log("Depth first iterative solution:")
alg1_output = depth_first_iterative_alg(a);
alg1_output.forEach(function(elem){
    console.log(elem.value);
});

console.log("\nDepth first recursive solution:")
alg2_output = depth_first_recursive_alg(a);
alg2_output.forEach(function(elem){
    console.log(elem.value);
});

console.log("\nBreadth first iterative solution:")
alg3_output = breadth_first_alg(a);
alg3_output.forEach(function(elem){
    console.log(elem.value);
});