/*** 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: A tree* @return: buttom-up level order a list of lists of integer*/public List<List<Integer>> levelOrderBottom(TreeNode root) {// write your code hereif(root == null){return new ArrayList<List<Integer>>();}List<List<Integer>> ans = work(levelOrderBottom(root.left),levelOrderBottom(root.right));／／递归List<Integer> rootval = new ArrayList<Integer>();rootval.add(root.val);ans.add(rootval);return ans;}public List<List<Integer>> work(List<List<Integer>> left,List<List<Integer>>right){／／将两个二维数组 从数组下面开始往上合并（（1），（2，3）） （（6））结果是（（1），（2，3，6））if(left.size() == 0){return right;}if(right.size() == 0){return left;}int len = Math.max(right.size(),left.size());int count = 1;List<List<Integer>> ans = new ArrayList<List<Integer>>();for(int i =0 ;i<len;i++){／／java中array list 只可以add长度比size（）大一或者以内的，如果add一个新的 老的会被后挤。所以有了6行后的remove，要是有更好的方法，欢迎赐教（本人java小白一个）ans.add(i,new ArrayList<Integer>());}while(count<=len){List<Integer> lv = left.size() == 0?new ArrayList<Integer>():left.remove(left.size()-1);List<Integer> rv = right.size() == 0?new ArrayList<Integer>():right.remove(right.size()-1);ans.remove(len-count);ans.add(len-count,miniwork(lv,rv));count++;}return ans;}public List<Integer> miniwork(List<Integer> left,List<Integer> right){//作用合并两个一维数组，右边的拼到左边if(left.size() == 0){return right;}if(right.size() == 0){return left;}int len = right.size();int count = 0;while(count<len){left.add(right.get(count));count++;}return left;}
}