考察BFS与DFS
BFS代码:
从根节点开始处理,在取得本层的数据平均值时同时使用队列记录下一层的节点数据,无需构建新的数据结构
class Solution {
public List<Double> averageOfLevels(TreeNode root) {
List<Double> result = new ArrayList<Double>();
if(root == null) return result;
Queue<TreeNode> queue = new LinkedList<TreeNode>();
queue.offer(root);
while(!queue.isEmpty()){
int len = queue.size();
double sum = 0.0;
for(int i = 0 ; i < len ; i++){
TreeNode t = queue.poll();
sum += t.val;
if(t.left != null) queue.offer(t.left);
if(t.right != null) queue.offer(t.right);
}
result.add(sum/len);
}
return result;
}
}
DFS思想:
从根节点开始处理,先记录每层(每层对应其索引值)的开始节点数据,之后一个个加确定总和与节点个数,要使用递归并构造新的节点数据结构(总和与节点数量)
class Solution {
class Node {
double sum;
int count;
Node (double d, int c) {
sum = d;
count = c;
}
}
public List<Double> averageOfLevels(TreeNode root) {
List<Node> temp = new ArrayList<>();
helper(root, temp, 0);
List<Double> result = new LinkedList<>();
for (int i = 0; i < temp.size(); i++) {
result.add(temp.get(i).sum / temp.get(i).count);
}
return result;
}
public void helper(TreeNode root, List<Node> temp, int level) {
if (root == null) return;
if (level == temp.size()) {
Node node = new Node((double)root.val, 1);
temp.add(node);
} else {
temp.get(level).sum += root.val;
temp.get(level).count++;
}
helper(root.left, temp, level + 1);
helper(root.right, temp, level + 1);
}
}