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Lint69.java
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38 lines (36 loc) · 1.25 KB
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/**
* Definition of TreeNode:
* public class TreeNode {
* public int val;
* public TreeNode left, right;
* public TreeNode(int val) {
* this.val = val;
* this.left = this.right = null;
* }
* }
*/
public class Solution {
/**
* @param root: The root of binary tree.
* @return: Level order a list of lists of integer
*/
public ArrayList<ArrayList<Integer>> levelOrder(TreeNode root) {
// write your code here
//Key point:关键是要理解递归的过程及顺序
//广度遍历???
ArrayList<ArrayList<Integer>> list = new ArrayList<>();
helper(list,root,0);
return list;
}
public void helper(ArrayList<ArrayList<Integer>> list,TreeNode node,int depth){
//Key point:Recursion执行是自上至下的，有时候认为是自下至上，是因为那些algo需要处理return值->那自然就要自下而上的考虑了
//Key point:List中如果删除了元素，index会自动改变的。相当于动态数组
//所以如果depth如果不存在的话，会和arr一样报OutOfBound错误
if(node != null){
if(list.size()-1<depth) list.add(new ArrayList<Integer>());
list.get(depth).add(node.val);
helper(list,node.left,depth+1);
helper(list,node.right,depth+1);
}
}
}