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Implement a method to perform basic string compression using the counts of repeated characters. For example, the string aabcccccaaa would become a2blc5a3. If the “compressed” string would not become smaller than the original string, your method should return the original string. You can assume the string has only uppercase and lowercase letters (a - z).

Example 1:

Input: “aabcccccaaa”
Output: “a2b1c5a3”
Example 2:

Input: “abbccd”
Output: “abbccd”
Explanation:
The compressed string is “a1b2c2d1”, which is longer than the original string.

Note:

Reverse a singly linked list.

Example:

Input: 1->2->3->4->5->NULL
Output: 5->4->3->2->1->NULL
Follow up:

A linked list can be reversed either iteratively or recursively. Could you implement both?

来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/reverse-linked-list
著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。

1. 一个队列实现

push: 在队列后添加元素
pop: 弹出的实际上是队尾的元素，因此可以用另一个队列暂存前半部分，也可以直接将队列前半部分的元素弹出并添加到队尾，保持了顺序不变。
top: 同pop
pop和top操作需要移动整个队列，复杂度为O(n)。

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class MyStack {
public:
    /** Initialize your data structure here. */
    MyStack() {}
    
    /** Push element x onto stack. */
    void push(int x) {
        q.push(x);
    }
    
    /** Removes the element on top of the stack and returns that element. */
    int pop() {
        int n = q.size();
        for (int i = 1; i < n; i++) {
            q.push(q.front());
            q.pop();
        }
        int val = q.front();
        q.pop();

        return val;
    }
    
    /** Get the top element. */
    int top() {
        int val = pop();
        q.push(val);

        return val;
    }
    
    /** Returns whether the stack is empty. */
    bool empty() {
        return q.empty();
    }

private:
    queue<int> q;
};

/**
 * Your MyStack object will be instantiated and called as such:
 * MyStack* obj = new MyStack();
 * obj->push(x);
 * int param_2 = obj->pop();
 * int param_3 = obj->top();
 * bool param_4 = obj->empty();
 */

2. 复杂度互换

方法1中插入的复杂度是O(1)，弹出复杂度是O(n)。
如果pop和top操作较多，则复杂度较高。
可以在插入时就将队列的顺序转为栈的顺序，即插入元素后，将其前部分弹出并插入到队尾。
从而将插入的复杂度提高到O(n)，pop和top复杂度降低到O(1)。

Given a non-empty 2D array grid of 0’s and 1’s, an island is a group of 1’s (representing land) connected 4-directionally (horizontal or vertical.) You may assume all four edges of the grid are surrounded by water.

Find the maximum area of an island in the given 2D array. (If there is no island, the maximum area is 0.)

Example 1:

[[0,0,1,0,0,0,0,1,0,0,0,0,0],
[0,0,0,0,0,0,0,1,1,1,0,0,0],
[0,1,1,0,1,0,0,0,0,0,0,0,0],
[0,1,0,0,1,1,0,0,1,0,1,0,0],
[0,1,0,0,1,1,0,0,1,1,1,0,0],
[0,0,0,0,0,0,0,0,0,0,1,0,0],
[0,0,0,0,0,0,0,1,1,1,0,0,0],
[0,0,0,0,0,0,0,1,1,0,0,0,0]]
Given the above grid, return 6. Note the answer is not 11, because the island must be connected 4-directionally.
Example 2:

[[0,0,0,0,0,0,0,0]]
Given the above grid, return 0.
Note: The length of each dimension in the given grid does not exceed 50.

Given an unsorted array of integers, find the length of longest increasing subsequence.

Example:

Input: [10,9,2,5,3,7,101,18]
Output: 4
Explanation: The longest increasing subsequence is [2,3,7,101], therefore the length is 4.
Note:

There may be more than one LIS combination, it is only necessary for you to return the length.
Your algorithm should run in O(n2) complexity.
Follow up: Could you improve it to O(n log n) time complexity?

来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/longest-increasing-subsequence
著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。

Given an array nums of n integers, are there elements a, b, c in nums such that a + b + c = 0? Find all unique triplets in the array which gives the sum of zero.

Note:

The solution set must not contain duplicate triplets.

Example:

Given array nums = [-1, 0, 1, 2, -1, -4],

Given an input string (s) and a pattern (p), implement regular expression matching with support for ‘.’ and ‘*’.

‘.’ Matches any single character.
‘*’ Matches zero or more of the preceding element.
The matching should cover the entire input string (not partial).

Note:

s could be empty and contains only lowercase letters a-z.
p could be empty and contains only lowercase letters a-z, and characters like . or *.
Example 1:

Input:
s = “aa”
p = “a”
Output: false
Explanation: “a” does not match the entire string “aa”.
Example 2:

There are two sorted arrays nums1 and nums2 of size m and n respectively.

Find the median of the two sorted arrays. The overall run time complexity should be O(log (m+n)).

You may assume nums1 and nums2 cannot be both empty.

Example 1:

nums1 = [1, 3]
nums2 = [2]

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 the two numbers and return it as a linked list.

You may assume the two numbers do not contain any leading zero, except the number 0 itself.

Example:

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8
Explanation: 342 + 465 = 807.

来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/add-two-numbers
著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。