construct-string-from-binary-tree.js

/* Construct String from Binary Tree

Given the root of a binary tree, 
construct a string consisting of parenthesis and integers from a binary tree with the preorder traversal way, and return it.
Omit all the empty parenthesis pairs that do not affect the one-to-one mapping relationship between the string and the original binary tree.

Example 1:
Input: root = [1,2,3,4]
Output: "1(2(4))(3)"
Explanation: Originally, it needs to be "1(2(4)())(3()())", but you need to omit all the unnecessary empty parenthesis pairs.
And it will be "1(2(4))(3)"

Example 2:
Input: root = [1,2,3,null,4]
Output: "1(2()(4))(3)"
Explanation: Almost the same as the first example,
except we cannot omit the first parenthesis pair to break the one-to-one mapping relationship between the input and the output. */

/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @return {string}
 */
 var tree2str = function(root) {
    
    if(root === null){
        return '';    
    }
    
    function preOrder(node){
        let res = '';
        
        if(node === null){
            return res;
        }
        
        res = `${node.val}`;
        
        if(node.left){
            res += `(${preOrder(node.left)})`;
        }
        
        if(node.right){
            if(!node.left){
                res += `()`;
            }
            res += `(${preOrder(node.right)})`;
        }
        
        return res;
    }
    
    return preOrder(root,'')
};