【树】Construct Binary Tree from Preorder and Inorder Traversal

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {number[]} preorder
 * @param {number[]} inorder
 * @return {TreeNode}
 */
var buildTree = function(preorder, inorder) {
    if(preorder.length==0){
        return null;
    }
    return BuildTree(0,preorder.length-1,0,inorder.length-1,preorder,inorder);
};

function BuildTree(pStart,pEnd,iStart,iEnd,preorder,inorder){
    if(pStart==pEnd){
        return new TreeNode(preorder[pStart]);
    }
    if(pStart>pEnd){
        return null;
    }
    var rootval=preorder[pStart];
    var i=inorder.indexOf(rootval);
    var root=new TreeNode(rootval);
    root.left=BuildTree(pStart+1,pStart+(i-iStart),iStart,i-1,preorder,inorder);
    root.right=BuildTree(pStart+1+(i-iStart),pEnd,i+1,iEnd,preorder,inorder);
    return root;
}