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Leetcode 261. Graph Valid Tree 无向图成环问题
题目大意 Given n nodes labeled from 0 to n-1 and a list of undirected edges (each edge is a pair of nodes), write a function to check whether these edges make up a valid tree. Example 1: Input: n = 5, and edges = [[0,1], …
Leetcode 101. Symmetric Tree 化用深度搜索
Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center). For example, this binary tree [1,2,2,3,4,4,3] is symmetric: 1 / \ 2 2 / \ / \ 3 4 4 3 But the following [1,2,2,null,3,null,3] is not: 1 …
Leetcode 279. Perfect Squares 经典一维动态规划问题
题目表述 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 …