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/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    private int bottomLeftValue = -1;
    private int maxDepth = 0;
    public int findBottomLeftValue(TreeNode root) {
        if (root == null) {
            return -1;
        }
        dfsHelper(root, 1);
        return bottomLeftValue;
    }
    
    private void dfsHelper(TreeNode root, int depth) {
        if (root == null) {
            return;
        }
        
        if (depth > maxDepth) {
            //update the result;
            maxDepth = depth;
            bottomLeftValue = root.val;
        }
        dfsHelper(root.left, depth + 1);
        dfsHelper(root.right, depth + 1);
    }
}

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public int findBottomLeftValue(TreeNode root) {
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        int left = -1;
        while (!queue.isEmpty()) {
            int curSize = queue.size();
            for (int i = 0; i < curSize; i++) {
                TreeNode cur = queue.poll();
                if (i == 0) left = cur.val;
                if (cur.left != null) queue.offer(cur.left);
                if (cur.right != null) queue.offer(cur.right);
            }
        }
        return left;
    }
}