Given two non-negative integers, num1 and num2 represented as string, return the sum of num1and num2 as a string.

You must solve the problem without using any built-in library for handling large integers (such as BigInteger). You must also not convert the inputs to integers directly.

Example 1:

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Input: num1 = "11", num2 = "123"
Output: "134"

Example 2:

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Input: num1 = "456", num2 = "77"
Output: "533"

Example 3:

Given an array of integers nums and an integer k, return the total number of continuous subarrays whose sum equals to k.

Example 1:

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Input: nums = [1,1,1], k = 2
Output: 2

Example 2:

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Input: nums = [1,2,3], k = 3
Output: 2

Constraints:

• 1 <= nums.length <= 2 * 104
• -1000 <= nums[i] <= 1000
• -107 <= k <= 107

解题思路：

累积和（使用数组）

Given a string s that contains parentheses and letters, remove the minimum number of invalid parentheses to make the input string valid.

Return all the possible results. You may return the answer in any order.

Example 1:

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Input: s = "()())()"
Output: ["(())()","()()()"]

Example 2:

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Input: s = "(a)())()"
Output: ["(a())()","(a)()()"]

Example 3:

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Input: s = ")("
Output: [""]

Constraints:

• 1 <= s.length <= 25
• s consists of lowercase English letters and parentheses '(' and ')'.
• There will be at most 20 parentheses in s.

难点：本题和1249. 移除无效的括号非常相似，但是这里最难的点是如何找到所有的可能解。

Given two binary strings a and b, return their sum as a binary string.

Example 1:

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Input: a = "11", b = "1"
Output: "100"

Example 2:

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Input: a = "1010", b = "1011"
Output: "10101"

Constraints:

• 1 <= a.length, b.length <= 104
• a and b consist only of '0' or '1' characters.
• Each string does not contain leading zeros except for the zero itself.

解题思路

直接按照进位处理即可。

Convert a non-negative integer num to its English words representation.

Example 1:

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Input: num = 123
Output: "One Hundred Twenty Three"

Example 2:

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Input: num = 12345
Output: "Twelve Thousand Three Hundred Forty Five"

Example 3:

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Input: num = 1234567
Output: "One Million Two Hundred Thirty Four Thousand Five Hundred Sixty Seven"

Constraints:

• 0 <= num <= 2^31 - 1

解题思路：

根据数字长度分成不同的翻译位:

Given an array of points where points[i] = [xi, yi] represents a point on the X-Y plane and an integer k, return the k closest points to the origin (0, 0).

The distance between two points on the X-Y plane is the Euclidean distance (i.e., √(x1 - x2)2 + (y1 - y2)2).

You may return the answer in any order. The answer is guaranteed to be unique(except for the order that it is in).