Leetcode#20. Valid Parentheses
ProblemGiven a string s containing just the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid.
An input string is valid if:
Open brackets must be closed by the same type of brackets.
Open brackets must be closed in the correct order.
Every close bracket has a corresponding open bracket of the same type.
Example 1:
123Input: s = "()"Output: true
Example 2:
123Input: s = "()[]{}&q ...
Leetcode#114. Flatten Binary Tree to Linked List
ProblemGiven the root of a binary tree, flatten the tree into a “linked list”:
The “linked list” should use the same TreeNode class where the right child pointer points to the next node in the list and the left child pointer is always null.
The “linked list” should be in the same order as a pre-order traversal of the binary tree.
Example 1:
!https://assets.leetcode.com/uploads/2021/01/14/flaten.jpg
123Input: root = [1,2,5,3,4,null,6]Output: [1,null,2,null,3,null,4,null,5,null,6]
Example 2:
1 ...
Leetcode#224. Basic Calculator
ProblemGiven a string s representing a valid expression, implement a basic calculator to evaluate it, and return the result of the evaluation.
Note: You are not allowed to use any built-in function which evaluates strings as mathematical expressions, such as eval().
Example 1:
123Input: s = "1 + 1"Output: 2
Example 2:
123Input: s = " 2-1 + 2 "Output: 3
Example 3:
123Input: s = "(1+(4+5+2)-3)+(6+8)"Output: 23
Constraints:
1 <= s.length <= 3 * 10^5
s consist ...