Oliver's Blog
Search
K

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++) {
res.add(levels.get(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<>());
levels.get(maxDepth).add(root.val);
return maxDepth;
}
}

Notes

  • 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.
  • Use a similar mechanism in Maximum Depth of Binary Tree and add a few operations to populate the map.
  • Lastly, convert the values in the map to a result list and return.