LintCode 650 · Find Leaves of Binary Tree (binary tree 后序遍历非常好的题目!)

650 · Find Leaves of Binary Tree
Algorithms
Medium
Description
Given a binary tree, collect a tree’s nodes as if you were doing this: Collect and remove all leaves, repeat until the tree is empty.

Example
Example1
Input: {1,2,3,4,5}
Output: [[4, 5, 3], [2], [1]].
Explanation:

1

/
2 3
/ \
4 5
Example2
Input: {1,2,3,4}
Output: [[4, 3], [2], [1]].
Explanation:

1

/
2 3
/
4
Tags
Company
LinkedIn
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155
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解法1：
这是一道非常好的题目！
用后序遍历！

/**
 * Definition of TreeNode:
 * class TreeNode {
 * public:
 *     int val;
 *     TreeNode *left, *right;
 *     TreeNode(int val) {
 *         this->val = val;
 *         this->left = this->right = NULL;
 *     }
 * }
 */


class Solution {
public:
    /*
     * @param root: the root of binary tree
     * @return: collect and remove all leaves
     */
    vector<vector<int>> findLeaves(TreeNode * root) {
        // write your code here
        helper(root);
        return res;
    }
private:
    vector<vector<int>> res;
    int helper(TreeNode *root) {
        if (!root) return 0;
//下面这段不需要
//        if (!root->left && !root->right) {
//            if (res.size() < 1) res.push_back(vector<int>());
//            res[0].push_back(root->val);
//            return 1;
//        }
        int left = helper(root->left);
        int right = helper(root->right);

        int height = max(left, right) + 1;
        if (res.size() < height) res.push_back(vector<int>());
        res[height - 1].push_back(root->val);
        return height;
    }
};