Leetcode Day 27 - Greedy: Advanced Interval & Sequence Problems

Greedy Algorithm

Diff	Problem	Python	Java
	56 Merge Intervals	✅
	738 Monotone Increasing Digits	✅
	968 Binary Tree Cameras

Merge Intervals

Link to Leetcode question¹

Given an array of intervals where intervals[i] = [starti, endi], merge all overlapping intervals, and return an array of the non-overlapping intervals that cover all the intervals in the input.

Example 1

Input: intervals = [[1,3],[2,6],[8,10],[15,18]]
Output: [[1,6],[8,10],[15,18]]
Explanation: Since intervals [1,3] and [2,6] overlap, merge them into [1,6].

Example 2

Input: intervals = [[1,4],[4,5]]
Output: [[1,5]]
Explanation: Intervals [1,4] and [4,5] are considered overlapping.

Solution

A detailed explaination of solution can be found here².

Python

class Solution(object):
    def merge(self, intervals):

        merged = []

        for start, end in sorted(intervals, key=lambda i: i[0]):
            if len(merged) != 0 and start <= merged[-1][1]:
                merged[-1][1] = max(end, merged[-1][1])
            else:
                merged.append([start, end])
        
        return merged

Similar Questions

Diff	Similar Questions	Python	Java
	57 Insert Interval3
	715 Range Module4

Monotone Increasing Digits

Link to Leetcode question⁵

An integer has monotone increasing digits if and only if each pair of adjacent digits x and y satisfy x <= y.

Given an integer n, return the largest number that is less than or equal to n with monotone increasing digits.

Example 1

Input: n = 10
Output: 9

Example 2

Input: n = 1234
Output: 1234

Example 3

Input: n = 332
Output: 299

Solution

A detailed explaination of solution can be found here⁶.

Python

class Solution(object):
    def monotoneIncreasingDigits(self, n):
        ln = list(str(n))

        if ln == sorted(ln):
            return n
        
        for i in range(len(ln) - 1, 0, -1):
            if ln[i] < ln[i - 1]:
                ln[i - 1] = str(int(ln[i - 1]) - 1)

                for j in range(i, len(ln)):
                    ln[j] = '9'
        
        return int(''.join(ln))

Similar Questions

Diff	Similar Questions	Python	Java
	402 Remove K Digits[^dtmnoall]

Binary Tree Cameras

Link to Leetcode question⁷

You are given the root of a binary tree. We install cameras on the tree nodes where each camera at a node can monitor its parent, itself, and its immediate children.

Return the minimum number of cameras needed to monitor all nodes of the tree.

Example 1

Input: root = [0,0,null,0,0]
Output: 1
Explanation: One camera is enough to monitor all nodes if placed as shown.

Example 2

Input: root = [0,0,null,0,null,0,null,null,0]
Output: 2
Explanation: At least two cameras are needed to monitor all nodes of the tree. The above image shows one of the valid configurations of camera placement.

Solution

A detailed explaination of solution can be found here⁸.

Similar Questions

Diff	Similar Questions	Python	Java
	979 Distribute Coins in Binary Tree9

Reference

1. Leetcode-56 Merge Intervals: https://leetcode.com/problems/merge-intervals/description/. ↩︎

2. 代码随想录-合并区间: https://programmercarl.com/0056.合并区间.html. ↩︎

3. Leetcode-57 Insert Interval: https://leetcode.com/problems/insert-interval/description/. ↩︎

4. Leetcode-715 Range Module: https://leetcode.com/problems/range-module/. ↩︎

5. Leetcode-738 Monotone Increasing Digits: https://leetcode.com/problems/monotone-increasing-digits/description/. ↩︎

6. 代码随想录-单调递增的数字: https://programmercarl.com/0738.单调递增的数字.html. ↩︎

7. Leetcode-968 Binary Tree Cameras: https://leetcode.com/problems/binary-tree-cameras/description/. ↩︎

8. 代码随想录-监控二叉树: https://programmercarl.com/0968.监控二叉树.html. ↩︎

9. Leetcode-979 Distribute Coins in Binary Tree: https://leetcode.com/problems/distribute-coins-in-binary-tree/. ↩︎