105. Construct-Binary-Tree-From-Preorder-And-Inorder-Traversal

/**
 * 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 {number[]} preorder
 * @param {number[]} inorder
 * @return {TreeNode}
 */
var buildTree = function(preorder, inorder) {
    let n = preorder.length;

    function helper(preLeft, preRight, inLeft, inRight){
        if(preLeft > preRight || inLeft > inRight)
            return null;
        let root = new TreeNode(preorder[preLeft]);
        let pivot = inLeft;
        while(inorder[pivot] !== root.val) pivot++;
        let length = pivot - inLeft;
        console.log(length)
        root.left = helper(preLeft+1,preLeft+length,inLeft,pivot-1);
        root.right = helper(preLeft + length + 1, preRight, pivot+1,inRight);
        return root;
    }

    return helper(0,n-1,0,n-1); 
};