Tree, Medium

Question

Given a binary tree

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5

struct TreeLinkNode {
  TreeLinkNode *left;
  TreeLinkNode *right;
  TreeLinkNode *next;
}

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.

Note:

• You may only use constant extra space.
• Recursive approach is fine, implicit stack space does not count as extra space for this problem.
• You may assume that it is a perfect binary tree (ie, all leaves are at the same level, and every parent has two children).

Example:

Given the following perfect binary tree,

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5

     1
   /  \
  2    3
 / \  / \
4  5  6  7

After calling your function, the tree should look like:

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3
4
5

     1 -> NULL
   /  \
  2 -> 3 -> NULL
 / \  / \
4->5->6->7 -> NULL

Tree, Medium

Question

Given a binary tree, find the leftmost value in the last row of the tree.

Example 1:

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8

Input:
    2
   / \
  1   3
Output:
1

Example 2:

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7
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9
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12

Input:
        1
       / \
      2   3
     /   / \
    4   5   6
       /
      7
Output:
7

Note: You may assume the tree (i.e., the given root node) is not NULL.

Tree, Medium

Question

Given a binary tree, imagine yourself standing on the right side of it, return the values of the nodes you can see ordered from top to bottom.

Example:

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9

Input: [1,2,3,null,5,null,4]
Output: [1, 3, 4]
Explanation:
   1            <---
 /   \
2     3         <---
 \     \
  5     4       <---

Question

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],

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5

  3
 / \
9  20
  /  \
 15   7

return its bottom-up level order traversal as:

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

Tree, Medium

Question

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

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

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2
3
4
5

  3
 / \
9  20
  /  \
 15   7

return its level order traversal as:

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5

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

Question

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

Example:

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8

Input: [1,null,2,3]
   1
    \
     2
    /
   3
Output: [3,2,1]

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

Question

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

Example:

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8

Input: [1,null,2,3]
   1
    \
     2
    /
   3
Output: [1,3,2]

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

Question

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

Example:

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5
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7
8

Input: [1,null,2,3]
   1
    \
     2
    /
   3
Output: [1,2,3]

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

Question

Given n non-negative integers representing the histogram’s bar height where the width of each bar is 1, find the area of largest rectangle in the histogram.

Above is a histogram where width of each bar is 1, given height = [2,1,5,6,2,3].

The largest rectangle is shown in the shaded area, which has area = 10 unit.

Example:

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2

Input: [2,1,5,6,2,3]
Output: 10