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Posts tagged as “hard”

花花酱 LeetCode 42. Trapping Rain Water

Given n non-negative integers representing an elevation map where the width of each bar is 1, compute how much water it is able to trap after raining.


The above elevation map is represented by array [0,1,0,2,1,0,1,3,2,1,2,1]. In this case, 6 units of rain water (blue section) are being trapped. Thanks Marcos for contributing this image!

Example:

Input: [0,1,0,2,1,0,1,3,2,1,2,1]
Output: 6

Solution 1: Brute Force

r[i] = min(max(h[0:i+1]), max(h[i:n]))
ans = sum(r[i])

Time complexity: O(n^2)
Space complexity: O(1)

C++

Solution 2: DP

l[i] := max(h[0:i+1])
r[i] := max(h[i:n])
ans = sum(min(l[i], r[i]) – h[i])

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

C++

Solution 3: Two Pointers

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

C++

花花酱 LeetCode 1320. Minimum Distance to Type a Word Using Two Fingers

You have a keyboard layout as shown above in the XY plane, where each English uppercase letter is located at some coordinate, for example, the letter A is located at coordinate (0,0), the letter B is located at coordinate (0,1), the letter P is located at coordinate (2,3) and the letter Z is located at coordinate (4,1).

Given the string word, return the minimum total distance to type such string using only two fingers. The distance between coordinates (x1,y1) and (x2,y2) is |x1 – x2| + |y1 – y2|

Note that the initial positions of your two fingers are considered free so don’t count towards your total distance, also your two fingers do not have to start at the first letter or the first two letters.

Example 1:

Input: word = "CAKE"
Output: 3
Explanation: 
Using two fingers, one optimal way to type "CAKE" is: 
Finger 1 on letter 'C' -> cost = 0 
Finger 1 on letter 'A' -> cost = Distance from letter 'C' to letter 'A' = 2 
Finger 2 on letter 'K' -> cost = 0 
Finger 2 on letter 'E' -> cost = Distance from letter 'K' to letter 'E' = 1 
Total distance = 3

Example 2:

Input: word = "HAPPY"
Output: 6
Explanation: 
Using two fingers, one optimal way to type "HAPPY" is:
Finger 1 on letter 'H' -> cost = 0
Finger 1 on letter 'A' -> cost = Distance from letter 'H' to letter 'A' = 2
Finger 2 on letter 'P' -> cost = 0
Finger 2 on letter 'P' -> cost = Distance from letter 'P' to letter 'P' = 0
Finger 1 on letter 'Y' -> cost = Distance from letter 'A' to letter 'Y' = 4
Total distance = 6

Example 3:

Input: word = "NEW"
Output: 3

Example 4:

Input: word = "YEAR"
Output: 7

Constraints:

  • 2 <= word.length <= 300
  • Each word[i] is an English uppercase letter.

Solution: DP

Top down: O(n*27^2)

C++

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C++

花花酱 LeetCode 1316. Distinct Echo Substrings

Return the number of distinct non-empty substrings of text that can be written as the concatenation of some string with itself.

Example 1:

Input: text = "abcabcabc"
Output: 3
Explanation: The 3 substrings are "abcabc", "bcabca" and "cabcab".

Example 2:

Input: text = "leetcodeleetcode"
Output: 2
Explanation: The 2 substrings are "ee" and "leetcodeleetcode".

Constraints:

  • 1 <= text.length <= 2000
  • text has only lowercase English letters.

Solution 1: Brute Force + HashSet

Try all possible substrings

Time complexity: O(n^3)
Space complexity: O(n^2)

C++

花花酱 LeetCode 1312. Minimum Insertion Steps to Make a String Palindrome

Given a string s. In one step you can insert any character at any index of the string.

Return the minimum number of steps to make s palindrome.

Palindrome String is one that reads the same backward as well as forward.

Example 1:

Input: s = "zzazz"
Output: 0
Explanation: The string "zzazz" is already palindrome we don't need any insertions.

Example 2:

Input: s = "mbadm"
Output: 2
Explanation: String can be "mbdadbm" or "mdbabdm".

Example 3:

Input: s = "leetcode"
Output: 5
Explanation: Inserting 5 characters the string becomes "leetcodocteel".

Example 4:

Input: s = "g"
Output: 0

Example 5:

Input: s = "no"
Output: 1

Constraints:

  • 1 <= s.length <= 500
  • All characters of s are lower case English letters.

Solution: DP

dp[i][j] := min chars to insert
dp[j][j] = dp[i-1][j+1] if s[i] == s[j] else min(dp[i+1][j] , dp[i][j-1]) + 1
base case: dp[i][i] = 0
ans: dp[0][n-1]

Time complexity: O(n^2)
Space complexity: O(n^2)

C++

花花酱 LeetCode 1301. Number of Paths with Max Score

You are given a square board of characters. You can move on the board starting at the bottom right square marked with the character 'S'.

You need to reach the top left square marked with the character 'E'. The rest of the squares are labeled either with a numeric character 1, 2, ..., 9 or with an obstacle 'X'. In one move you can go up, left or up-left (diagonally) only if there is no obstacle there.

Return a list of two integers: the first integer is the maximum sum of numeric characters you can collect, and the second is the number of such paths that you can take to get that maximum sum, taken modulo 10^9 + 7.

In case there is no path, return [0, 0].

Example 1:

Input: board = ["E23","2X2","12S"]
Output: [7,1]

Example 2:

Input: board = ["E12","1X1","21S"]
Output: [4,2]

Example 3:

Input: board = ["E11","XXX","11S"]
Output: [0,0]

Constraints:

  • 2 <= board.length == board[i].length <= 100

Solution: DP

dp[i][j] := max score when reach (j, i)
count[i][j] := path to reach (j, i) with max score

m = max(dp[i + 1][j], dp[i][j+1], dp[i+1][j+1])
dp[i][j] = board[i][j] + m
count[i][j] += count[i+1][j] if dp[i+1][j] == m
count[i][j] += count[i][j+1] if dp[i][j+1] == m
count[i][j] += count[i+1][j+1] if dp[i+1][j+1] == m

Time complexity: O(n^2)
Space complexity: O(n^2)

C++