605. Can Place Flowers

Description

Suppose 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 - they would compete for water and both would die.

Given a flowerbed (represented as an array containing 0 and 1, where 0 means empty and 1 means not empty), and a number n, return if n new flowers can be planted in it without violating the no-adjacent-flowers rule.

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

Note:
The input array won't violate no-adjacent-flowers rule.
The input array size is in the range of [1, 20000].
n is a non-negative integer which won't exceed the input array size.

Solution

Greedy, time O(n), space O(1)

class Solution {
    public boolean canPlaceFlowers(int[] flowerbed, int n) {
        if (n < 1) {
            return true;
        }
        
        int places = 0;
        for (int i = 0; i < flowerbed.length && places < n; ++i) {
            // If the place is empty, get next and prev flower bed slot values.
            // If i lies at the ends the next and prev are considered as 0.
            if (flowerbed[i] == 0 
                && (i == 0 || flowerbed[i - 1] == 0) 
                && (i == flowerbed.length - 1 || flowerbed[i + 1] == 0)) {
                flowerbed[i] = 1;
                ++places;
            }
        }
        
        return places == n; 
    }
}