Recover a Tree From Preorder Traversal.js

var recoverFromPreorder = function(traversal) {
    
    let n = traversal.length;
    
    // Every layer in dfs handles the depth+1 of '-' only.
    // ex: 
    //   depth=0 -> find '-'  as splitter
    //   depth=1 -> find '--' as splitter
    //   depth=2 -> find '---' as splitter
    let dfs = (str,depth)=>{
        if(str.indexOf("-") === -1) return new TreeNode(str);
		
		// 1. We split by the depth+1 number of '-'
        // Using regex to split is much easier. -> str.split(/(?<=\d)-(?=\d)/g)
		// where (?<=\d)  means positive lookbehind , ex: "1- ...", then we'll split '-' excluding 1.
		// Similarly , (?=\d) means positive lookahead , ex: "-5 ...",  then we'll split '-' excluding 5.
		
        let re = new RegExp(`(?<=\\d)${"-".repeat(depth+1)}(?=\\d)`,'g');
        let [val,leftStr,rightStr] = str.split(re);
        // ex: 1-2--3--4-5--6--7 --> ['1','2--3--4','5--6--7']
        
		// 2. After splitting,  we'll get [val,leftStr,rightStr]
		// Then we could handle left / right node in the next dfs layer intuitively.
        let node = new TreeNode(val);
        if(leftStr)  node.left  = dfs(leftStr,depth+1);
        if(rightStr) node.right = dfs(rightStr,depth+1);
        
        return node;
    };
    
    return dfs(traversal,0);
    
};