js遍历二叉树算法

function TreeNode(val) {
    this.val = val;
    this.left = this.right = null;
}

function preorderTraversal(root) {
    let result = [];
    function traverse(node) {
        if (node === null) {
            return;
        }
        result.push(node.val);
        traverse(node.left);
        traverse(node.right);
    }
    traverse(root);
    return result;
}

function inorderTraversal(root) {
    let result = [];
    function traverse(node) {
        if (node === null) {
            return;
        }
        traverse(node.left);
        result.push(node.val);
        traverse(node.right);
    }
    traverse(root);
    return result;
}

function postorderTraversal(root) {
    let result = [];
    function traverse(node) {
        if (node === null) {
            return;
        }
        traverse(node.left);
        traverse(node.right);
        result.push(node.val);
    }
    traverse(root);
    return result;
}

function levelOrderTraversal(root) {
    if (root === null) {
        return [];
    }
    let result = [];
    let queue = [root];
    while (queue.length > 0) {
        let node = queue.shift();
        result.push(node.val);
        if (node.left!== null) {
            queue.push(node.left);
        }
        if (node.right!== null) {
            queue.push(node.right);
        }
    }
    return result;
}