摘要： Given a string which contains only lowercase letters, remove duplicate letters so that every letter appear once and only once. You must make sure your 阅读全文

摘要： Given an integer array with no duplicates. A maximum tree building on this array is defined as follow: Construct the maximum tree by the given array a 阅读全文

摘要： Given a Binary Search Tree and a target number, return true if there exist two elements in the BST such that their sum is equal to the given target. E 阅读全文