Find Leaves of Binary Tree

ID: 650; medium; 二叉树叶子顺序遍历

Solution 1 (Java)

``````/**
* 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) {
List<List<Integer>> res = new ArrayList<>();
Map<Integer, List<Integer>> levels = new HashMap<>();

int maxDepth = dfs(root, levels);
for (int i = 1; i <= maxDepth; i++) {
}
return res;
}

private int dfs(TreeNode root, Map<Integer, List<Integer>> levels) {
if (root == null) return 0;
int maxDepth = Math.max(dfs(root.left, levels), dfs(root.right, levels)) + 1;
// 1 is the bottom level
levels.putIfAbsent(maxDepth, new ArrayList<>());
• Use a `HashMap` to store the levels of the tree. The key is the level (1 is the bottom level) and the value of a list of node values.