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Posts tagged as “traversal”

花花酱 LeetCode 117. Populating Next Right Pointers in Each Node II

Given a binary tree

Populate each next pointer to point to its next right node. If there is no next right node, the next pointer should be set to NULL.

Initially, all next pointers are set to NULL.

Example 1:

Input: root = [1,2,3,4,5,null,7]
Output: [1,#,2,3,#,4,5,7,#]
Explanation: Given the above binary tree (Figure A), your function should populate each next pointer to point to its next right node, just like in Figure B. The serialized output is in level order as connected by the next pointers, with '#' signifying the end of each level.

Example 2:

Input: root = []
Output: []


  • The number of nodes in the tree is in the range [0, 6000].
  • -100 <= Node.val <= 100


  • You may only use constant extra space.
  • The recursive approach is fine. You may assume implicit stack space does not count as extra space for this problem.

Solution -2: Group nodes into levels

Use pre-order traversal to group nodes by levels.
Connects nodes in each level.

Time complexity: O(n)
Space complexity: O(n)


Solution -1: BFS level order traversal

Time complexity: O(n)
Space complexity: O(n)


Solution 1: BFS w/o extra space

Populating the next level while traversing current level.

Time complexity: O(n)
Space complexity: O(1)


花花酱 LeetCode 144. Binary Tree Preorder Traversal

Given a binary tree, return the preorder traversal of its nodes’ values.


Input: [1,null,2,3]

Output: [1,2,3]

Follow up: Recursive solution is trivial, could you do it iteratively?

Solution 1: Recursion

Time complexity: O(n)
Space complexity: O(n)


Solution 2: Stack

Time complexity: O(n)
Space complexity: O(n)


花花酱 LeetCode 1305. All Elements in Two Binary Search Trees

Given two binary search trees root1 and root2.

Return a list containing all the integers from both trees sorted in ascending order.

Example 1:

Input: root1 = [2,1,4], root2 = [1,0,3]
Output: [0,1,1,2,3,4]

Example 2:

Input: root1 = [0,-10,10], root2 = [5,1,7,0,2]
Output: [-10,0,0,1,2,5,7,10]

Example 3:

Input: root1 = [], root2 = [5,1,7,0,2]
Output: [0,1,2,5,7]

Example 4:

Input: root1 = [0,-10,10], root2 = []
Output: [-10,0,10]

Example 5:

Input: root1 = [1,null,8], root2 = [8,1]
Output: [1,1,8,8]


  • Each tree has at most 5000 nodes.
  • Each node’s value is between [-10^5, 10^5].

Solution: Inorder traversal + Merge Sort

Time complexity: O(t1 + t2)
Space complexity: O(t1 + t2)



花花酱 LeetCode 987. Vertical Order Traversal of a Binary Tree

Given a binary tree, return the vertical order traversal of its nodes values.

For each node at position (X, Y), its left and right children respectively will be at positions (X-1, Y-1) and (X+1, Y-1).

Running a vertical line from X = -infinity to X = +infinity, whenever the vertical line touches some nodes, we report the values of the nodes in order from top to bottom (decreasing Y coordinates).

If two nodes have the same position, then the value of the node that is reported first is the value that is smaller.

Return an list of non-empty reports in order of X coordinate.  Every report will have a list of values of nodes.

Example 1:

Input: [3,9,20,null,null,15,7]
Output: [[9],[3,15],[20],[7]]
Without loss of generality, we can assume the root node is at position (0, 0):
Then, the node with value 9 occurs at position (-1, -1);
The nodes with values 3 and 15 occur at positions (0, 0) and (0, -2);
The node with value 20 occurs at position (1, -1);
The node with value 7 occurs at position (2, -2).

Example 2:

Input: [1,2,3,4,5,6,7]
Output: [[4],[2],[1,5,6],[3],[7]]
The node with value 5 and the node with value 6 have the same position according to the given scheme.
However, in the report "[1,5,6]", the node value of 5 comes first since 5 is smaller than 6.


  1. The tree will have between 1 and 1000 nodes.
  2. Each node’s value will be between 0 and 1000.

Solution: Ordered Map+ Ordered Set

Time complexity: O(nlogn)
Space complexity: O(n)



花花酱 LeetCode 971. Flip Binary Tree To Match Preorder Traversal

Given a binary tree with N nodes, each node has a different value from {1, ..., N}.

A node in this binary tree can be flipped by swapping the left child and the right child of that node.

Consider the sequence of N values reported by a preorder traversal starting from the root.  Call such a sequence of N values the voyage of the tree.

(Recall that a preorder traversal of a node means we report the current node’s value, then preorder-traverse the left child, then preorder-traverse the right child.)

Our goal is to flip the least number of nodes in the tree so that the voyage of the tree matches the voyagewe are given.

If we can do so, then return a list of the values of all nodes flipped.  You may return the answer in any order.

If we cannot do so, then return the list [-1].

Example 1:

Input: root = [1,2], voyage = [2,1]
Output: [-1]

Example 2:

Input: root = [1,2,3], voyage = [1,3,2]
Output: [1]

Example 3:

Input: root = [1,2,3], voyage = [1,2,3]
Output: []


  1. 1 <= N <= 100

Solution: Pre-order traversal

if root->val != v[pos] return [-1]
if root->left?->val != v[pos + 1], swap the nodes