Problem

Given an index k, return the kth row of the Pascal’s triangle.

For example, given k = 3, Return [1,3,3,1].

Note: Could you optimize your algorithm to use only O(k) extra space?

即直接返回杨辉三角形的某一行元素，额外要求是只让使用O（k）的空间

学习TensorFlow过程中读的一篇文章，索性就翻译出来，虽然收获感觉不是很大。

原文地址：How TensorFlow Works

https://github.com/nfmcclure/tensorflow_cookbook/blob/master/01_Introduction/01_How_TensorFlow_Works/01_How_TensorFlow_Works.ipynb

Problem

Given numRows, generate the first numRows of Pascal’s triangle.

For example, given numRows = 5, Return

[
     [1],
    [1,1],
   [1,2,1],
  [1,3,3,1],
 [1,4,6,4,1]
]

没记错的话，这个应该叫杨辉三角型。很简单的题

Problem

Given a binary tree and a sum, find all root-to-leaf paths where each path’s sum equals the given sum.

For example: Given the below binary tree and sum = 22,

              5
             / \
            4   8
           /   / \
          11  13  4
         /  \    / \
        7    2  5   1

return

[
   [5,4,11,2],
   [5,8,4,5]
]

即Path Sum问题的进一步，不仅仅是判断是否存在路径，而是找出所有的路径。

Problem

Given a binary tree and a sum, determine if the tree has a root-to-leaf path such that adding up all the values along the path equals the given sum.

For example: Given the below binary tree and sum = 22,

              5
             / \
            4   8
           /   / \
          11  13  4
         /  \      \
        7    2      1

return true, as there exist a root-to-leaf path 5->4->11->2 which sum is 22.

class TreeNode:
    def __init__(self, x):
        self.val = x
        self.left = None
        self.right = None

Problem

Given a binary tree, find its minimum depth.

The minimum depth is the number of nodes along the shortest path from the root node down to the nearest leaf node.

class TreeNode:
    def __init__(self, x):
        self.val = x
        self.left = None
        self.right = None

Problem

Given a binary tree, determine if it is height-balanced.

For this problem, a height-balanced binary tree is defined as a binary tree in which the depth of the two subtrees of every node never differ by more than 1.

即判断一个二叉树是不是平衡树。平衡的标准就是任何节点的左右子树的高度差不大于1。

Problem

Given an array where elements are sorted in ascending order, convert it to a height balanced BST.

public class TreeNode {
        int val;
        TreeNode left;
        TreeNode right;

        TreeNode(int x) {
            val = x;
        }
    }

Problem

Given a binary tree, return the bottom-up level order traversal of its nodes’ values. (ie, from left to right, level by level from leaf to root).

For example: Given binary tree [3,9,20,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7

return its bottom-up level order traversal as:

[
  [15,7],
  [9,20],
  [3]
]

按层，从叶子到根输出树种每层的元素。树的定义如下：

public class TreeNode {
        int val;
        TreeNode left;
        TreeNode right;

        TreeNode(int x) {
            val = x;
        }
    }

Problem

Given a binary tree, find its maximum depth.

The maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.

求二叉树的最大深度