LintCode：650. Find Leaves of Binary Tree

题目：

分析：遍历二叉树，找到叶子结点后，将其加入到list中，然后将叶节点置为null，重复，直到root=null为止。这样可以一层层剥离开来。

/**
 * 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: collect and remove all leaves
     */
    public List<List<Integer>> findLeaves(TreeNode root) {
        // write your code here
        List<List<Integer>> res=new ArrayList<>();
        if(root==null)  return res;
        while(root!=null){
            List<Integer>  arr=new ArrayList<>();
            root=getLeaves(root,arr);
            res.add(arr);
        }
        return res;
    }

    public TreeNode getLeaves(TreeNode root,List<Integer> list){
        if(root==null)  return null;
        if(root.left==null && root.right==null){
            list.add(root.val);
            return null;
        }
        root.left=getLeaves(root.left,list);
        root.right=getLeaves(root.right,list);
        return root;
    }
}