LeetCode 513. Find Bottom Left Tree Value

原题链接在这里：https://leetcode.com/problems/find-bottom-left-tree-value/description/

题目：

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.

题解：

做从右到左的BFS.

Time Complexity: O(n). n是node个数.

Space: O(n).

AC Java:

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public int findBottomLeftValue(TreeNode root) {
        LinkedList<TreeNode> que = new LinkedList<TreeNode>();
        que.add(root);
        while(!que.isEmpty()){
            root = que.poll();
            if(root.right != null){
                que.add(root.right);
            }
            if(root.left != null){
                que.add(root.left);
            }
        }
        return root.val;
    }
}