Leetcode

题目： Design a stack that supports push, pop, top, and retrieving the minimum element in constant time. push(x) – Push element x onto stack. pop() – Removes the element on top of the stack. top() – Get the top element. getMin() – Retrieve the minimum element in the stack. Example: 1 2 3 4 5 6 7 8 MinStack minStack = new MinStack(); minStack.push(-2); minStack.push(0); minStack.push(-3); minStack.getMin(); --> Returns -3. minStack.pop(); minStack.top(); --> Returns 0. minStack.getMin(); --> Returns -2. 解题： 与普通栈不同，增加了一个返回栈最小值的功能， 这里使用了两个栈， 一个保存数据 ， 一个保存压栈过程中出现过的最小值。 ...

题目： Given a positive integer n, find the least number of perfect square numbers (for example, 1, 4, 9, 16, ...) which sum to n. Example 1: Input: n = 12 Output: 3 Explanation: 12 = 4 + 4 + 4. Example 2: Input: n = 13 Output: 2 Explanation: 13 = 4 + 9. 解题1： 数学方式的解法：基于四平方和理论（Lagrange’s four-square theorem）， 每个正整数均可表示为4个整数的平方和。 1. 如果整数包含因子4， 可以去掉不影响结果； 2. 如果除8余7的话， 肯定是由四个完全平方数组成； 3. 之后按照两个数的平方和拆解整数， 其中！！逻辑取反判断是否正整数还是0， 正整数！！后为1， 0！！后为0； ...

题目： You have a lock in front of you with 4 circular wheels. Each wheel has 10 slots: '0', '1', '2', '3', '4', '5', '6', '7', '8', '9'. The wheels can rotate freely and wrap around: for example we can turn '9' to be '0', or '0' to be '9'. Each move consists of turning one wheel one slot. The lock initially starts at '0000', a string representing the state of the 4 wheels. ...

题目： Given a linked list, rotate the list to the right by k places, where k is non-negative. Example 1: 1 2 3 4 5 Input: 1->2->3->4->5->NULL, k = 2 Output: 4->5->1->2->3->NULL Explanation: rotate 1 steps to the right: 5->1->2->3->4->NULL rotate 2 steps to the right: 4->5->1->2->3->NULL Example 2: Input: 0->1->2->NULL, k = 4 Output: 2->0->1->NULL Explanation: rotate 1 steps to the right: 2->0->1->NULL rotate 2 steps to the right: 1->2->0->NULL rotate 3 steps to the right: 0->1->2->NULL rotate 4 steps to the right: 2->0->1->NULL ...

A linked list is given such that each node contains an additional random pointer which could point to any node in the list or null

题目： You are given a doubly linked list which in addition to the next and previous pointers, it could have a child pointer, which may or may not point to a separate doubly linked list. These child lists may have one or more children of their own, and so on, to produce a multilevel data structure, as shown in the example below. Flatten the list so that all the nodes appear in a single-level, doubly linked list. You are given the head of the first level of the list. ...

You are given two non-empty linked lists representing two non-negative integers. The digits are stored in reverse order and each of their nodes contain a single digit. Add