513. Find Bottom Left Tree Value

BFS的变式 ，用一个result保存每一层的第一个就好了

/**
 * 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) {
        ArrayDeque<TreeNode> queue = new ArrayDeque<>();
        queue.add(root);
        int result = 0 ;
        while(!queue.isEmpty())
        {
            int size = queue.size();
            result = queue.peek().val;
            for(int i = 0 ;i<size;i++)
            {
                TreeNode node = queue.poll();
                if(node.left!=null)
                    queue.add(node.left);
                if(node.right!=null)
                    queue.add(node.right);
            }
        }
        return result ;
    }
}