Can Replace Flowers

Problem

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 Solution: O(n) time, O(1) space

public class Solution {
    public boolean canPlaceFlowers(int[] flowerbed, int n) {
        if (flowerbed == null || n > flowerbed.length / 2 + 1) return false;
        int count = 0;
        for (int i = 0; i < flowerbed.length && count < n; i++) {
            if (flowerbed[i] == 0) {
                int prev = i == 0 ? 0 : flowerbed[i - 1];
                int next = i == flowerbed.length - 1 ? 0 : flowerbed[i + 1];
                if (prev == 0 && next == 0) {
                    flowerbed[i] = 1;
                    count++;
                }
            }
        }
        return count == n;
    }
}

Analysis

This solution literally checks each position in flowerbed and increase count if there is available place
Therefore, we just need to return count == 0 at the end
To check if position i is available, we should see its previous i-1 and next i+1 in given flowerbed
If available, we first set flowerbed[i] = 1 then increment our count
To terminate the loop earlier, we add count < n inside for loop heading