605. Can Place Flowers

1. Description

You have a long flowerbed in which some of the plots are planted, and some are not. However, flowers cannot be planted in adjacent plots.
Given an integer array flowerbed containing 0’s and 1’s, where 0 means empty and 1 means not empty, and an integer n, return if n new flowers can be planted in the flowerbed without violating the no-adjacent-flowers rule.

2. Example

Example 1

Input: flowerbed = [1,0,0,0,1], n = 1
Output: true

Example 2

Input: flowerbed = [1,0,0,0,1], n = 2
Output: false

3. Constraints

• 1 <= flowerbed.length <= $2 * 10^4$
• flowerbed[i] is 0 or 1.
• There are no two adjacent flowers in flowerbed.
• 0 <= n <= flowerbed.length

4. Solutions

Greedy

n = flowerbed.size()
Time complexity: O(n)
Space complexity: O(1)

class Solution {
public:
    bool canPlaceFlowers(const vector<int> &flowerbed, int n) {
        int valid_places = 0;
        if (flowerbed[0] == 0 && (flowerbed.size() == 1 || flowerbed[1] == 0)) {
            valid_places = 1;
        }

        for (int i = 2; i < flowerbed.size();) {
            if (flowerbed[i] == 0 && flowerbed[i - 1] == 0 &&
                (i + 1 == flowerbed.size() || flowerbed[i + 1] == 0)) {
                valid_places += 1;

                i += 2;
            } else {
                ++i;
            }

            if (valid_places >= n) {
                return true;
            }
        }

        return valid_places >= n;
    }
};