Posts tagged as “greedy”

花花酱 LeetCode 942. DI String Match

By zxi on November 18, 2018

Problem

Given a string S that only contains “I” (increase) or “D” (decrease), let N = S.length.

Return any permutation A of [0, 1, ..., N] such that for all i = 0, ..., N-1:

If S[i] == "I", then A[i] < A[i+1]
If S[i] == "D", then A[i] > A[i+1]

Example 1:

Input: "IDID"
Output: [0,4,1,3,2]

Example 2:

Input: "III"
Output: [0,1,2,3]

Example 3:

Input: "DDI"
Output: [3,2,0,1]

Note:

1 <= S.length <= 10000
S only contains characters "I" or "D".

Solution: Greedy

“I” -> use the smallest possible number

“D” -> use the largest possible number

Time complexity: O(n)

Space complexity: O(n)

C++

// Author: Huahua, 44 ms

class Solution {

public:

vector<int> diStringMatch(string S) {

const int n = S.length();

vector<int> ans;

int lo = 0;

int hi = n;

for (char c : S) {

if (c == 'I')

ans.push_back(lo++);

else

ans.push_back(hi--);

}

ans.push_back(lo);

return ans;

}

};

花花酱 LeetCode 55. Jump Game

By zxi on November 15, 2018

Problem

Given an array of non-negative integers, you are initially positioned at the first index of the array.

Each element in the array represents your maximum jump length at that position.

Determine if you are able to reach the last index.

Example 1:

Input: [2,3,1,1,4]
Output: true
Explanation: Jump 1 step from index 0 to 1, then 3 steps to the last index.

Example 2:

Input: [3,2,1,0,4]
Output: false
Explanation: You will always arrive at index 3 no matter what. Its maximum
             jump length is 0, which makes it impossible to reach the last index.

Solution: Greedy

If you can jump to i, then you can jump to at least i + nums[i].

Always jump as far as you can.

Keep tracking the farthest index you can jump to.

Init far = nums[0].

far = max(far, i + nums[i])

check far >= n – 1

ex 1 [2,3,1,1,4]

i nums[i] i + nums[i] far

- - - 2

0 2 2 2

1 3 4 4

2 1 3 4

3 1 4 4

4 4 8 8

ex 2 [3,2,1,0,4]

i nums[i] i + nums[i] far

- - - 3

0 3 3 3

1 2 3 3

2 1 3 3

3 0 3 3 <- can not reach index 4, break

C++

// Author: Huahua, running time 16 ms

class Solution {

public:

bool canJump(vector<int>& nums) {

const int n = nums.size();

int far = nums[0];

for (int i = 0; i < n; i++) {

if (i > far) break;

far = max(far, i + nums[i]);

}

return far >= n-1;

}

};

花花酱 LeetCode 910. Smallest Range II

By zxi on September 26, 2018

Problem

Given an array A of integers, for each integer A[i] we need to choose either x = -K or x = K, and add x to A[i] (only once).

After this process, we have some array B.

Return the smallest possible difference between the maximum value of B and the minimum value of B.

Example 1:

Input: A = [1], K = 0
Output: 0
Explanation: B = [1]

Example 2:

Input: A = [0,10], K = 2
Output: 6
Explanation: B = [2,8]

Example 3:

Input: A = [1,3,6], K = 3
Output: 3
Explanation: B = [4,6,3]

Note:

1 <= A.length <= 10000
0 <= A[i] <= 10000
0 <= K <= 10000

Solution: Greedy

Sort the array and compare adjacent numbers.

Time complexity: O(nlogn)

Space complexity: O(1)

C++

// Author: Huahua

class Solution {

public:

int smallestRangeII(vector<int>& A, int K) {

sort(begin(A), end(A));

int ans = A.back() - A.front();

for (int i = 1; i < A.size(); ++i) {

int l = min(A.front() + K, A[i] - K);

int h = max(A.back() - K, A[i - 1] + K);

ans = min(ans, h - l);

}

return ans;

}

};

Python3

# Author: Huahua

class Solution:

def smallestRangeII(self, A, K):

A.sort()

ans = A[-1] - A[0]

for a, b in zip(A[0:-1], A[1:]):

l = min(A[0] + K, b - K)

h = max(A[-1] - K, a + K)

ans = min(ans, h - l)

return ans

花花酱 LeetCode 455. Assign Cookies

By zxi on August 15, 2018

Problem

Assume you are an awesome parent and want to give your children some cookies. But, you should give each child at most one cookie. Each child i has a greed factor g_i, which is the minimum size of a cookie that the child will be content with; and each cookie j has a size s_j. If s_j >= g_i, we can assign the cookie j to the child i, and the child i will be content. Your goal is to maximize the number of your content children and output the maximum number.

Note:
You may assume the greed factor is always positive.
You cannot assign more than one cookie to one child.

Example 1:

Input: [1,2,3], [1,1]

Output: 1

Explanation: You have 3 children and 2 cookies. The greed factors of 3 children are 1, 2, 3. 
And even though you have 2 cookies, since their size is both 1, you could only make the child whose greed factor is 1 content.
You need to output 1.

Example 2:

Input: [1,2], [1,2,3]

Output: 2

Explanation: You have 2 children and 3 cookies. The greed factors of 2 children are 1, 2. 
You have 3 cookies and their sizes are big enough to gratify all of the children, 
You need to output 2.

Solution: Greedy + Two Pointers

Time complexity: O(mlogm + nlogn)

Space complexity: O(1)

// Author: Huahua

// Running time: 28 ms

class Solution {

public:

int findContentChildren(vector<int>& g, vector<int>& s) {

sort(begin(g), end(g));

sort(begin(s), end(s));

int ans = 0;

int j = 0;

for (int i = 0; i < g.size(); ++i) {

while (j < s.size() && s[j] < g[i]) {

++j;

}

if (j < s.size()) {

++ans;

++j;

continue;

}

return ans;

}

};

花花酱 LeetCode 885. Boats to Save People

By zxi on August 4, 2018

Problem

The i-th person has weight people[i], and each boat can carry a maximum weight of limit.

Each boat carries at most 2 people at the same time, provided the sum of the weight of those people is at most limit.

Return the minimum number of boats to carry every given person. (It is guaranteed each person can be carried by a boat.)

Example 1:

Input: people = [1,2], limit = 3
Output: 1
Explanation: 1 boat (1, 2)

Example 2:

Input: people = [3,2,2,1], limit = 3
Output: 3
Explanation: 3 boats (1, 2), (2) and (3)

Example 3:

Input: people = [3,5,3,4], limit = 5
Output: 4
Explanation: 4 boats (3), (3), (4), (5)

Note:

1 <= people.length <= 50000
1 <= people[i] <= limit <= 30000

Solution: Greedy + Two Pointers

Time complexity: O(nlogn)

Space complexity: O(1)

Put one heaviest guy and put the lightest guy if not full.

// Author: Huahua

// Running time: 80 ms

class Solution {

public:

int numRescueBoats(vector<int>& people, int limit) {

sort(people.rbegin(), people.rend());

int i = 0;

int j = people.size() - 1;

int ans = 0;

while (i <= j) {

++ans;

if (i == j) break;

if (people[i++] + people[j] <= limit) --j;

}

return ans;

}

};