605. Can Place Flowers

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:

1. The input array won't violate no-adjacent-flowers rule.

2. The input array size is in the range of [1, 20000].

3. n is a non-negative integer which won't exceed the input array size.

利用Greedy method，每遇到1就要跳兩步，因為兩個花不能放在相鄰位置，如果發現一個位置的前後都是空的，可以放花，放完之後也能跳兩步，因為剛放完花之後的下一格一定不能放花。不然就是一步一步往前。

// Greedy
bool canPlaceFlowers(vector<int>& flowerbed, int n) { // time: O(n); space: O(1)
    if (flowerbed.empty()) return false;
    if (n == 0) return true;
    int cnt = 0, i = 0;
    while (i < flowerbed.size()) {
        if (flowerbed[i] == 1) i += 2;
        else if ((i == 0 || flowerbed[i - 1] == 0) && (i == flowerbed.size() - 1 || flowerbed[i + 1] == 0)) {
            if (++cnt == n) return true;
            i += 2;
        } else ++i;
    }
    return false;
}