513. Find Bottom Left Tree Value

Given a binary tree, find the leftmost value in the last row of the tree.

Example 1:
Input:

    2   / \  1   3

Output:
1

Example 2:
Input:

    1   / \  2   3 /   / \4   5   6   /  7

Output:
7

Note: You may assume the tree (i.e., the given root node) is not NULL.

其实本题目的意思是找二叉树最底层的第一个叶子节点
思路1：用队列实现

int findBottomLeftValue(TreeNode* root) {    if (root == NULL)return 0;    queue<TreeNode *> temp;    int val = 0;    temp.push(root);     while (!temp.empty()){        int size = temp.size();        val = temp.front()->val;        while (size--){            TreeNode *p = temp.front();            temp.pop();            if (p->left)temp.push(p->left);            if (p->right)temp.push(p->right);        }    }    return val;}

思路2：递归

class Solution {private:    int height = 0, val = 0;public:    void helper(TreeNode *root, int depth){        if (root == NULL)return;        if (height < depth){            val = root->val;            height = depth;        }        helper(root->left, depth + 1);        helper(root->right, depth + 1);    }    int findBottomLeftValue(TreeNode* root) {        if (root == NULL)return 0;        helper(root, 1);        return val;    }};