6.12 找树左下角的值

513. 找树左下角的值

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public int findBottomLeftValue(TreeNode root) {
            Queue<TreeNode> q = new LinkedList<>();
            q.offer(root);
            TreeNode ans = root;
            while (!q.isEmpty()){
                int size = q.size();
                ans = q.peek();
                for (int i = 0; i < size; i++) {
                    TreeNode poll = q.poll();
                    if (poll.left != null) q.offer(poll.left);
                    if (poll.right != null) q.offer(poll.right);
                }
            }
            return ans.val;
    }
}

点我递归
如果需要遍历整棵树，递归函数就不能有返回值。如果需要遍历某一条固定路线，递归函数就一定要有返回值！
本题我们是要遍历整个树找到最深的叶子节点，需要遍历整棵树，所以递归函数没有返回值

// 递归法
class Solution {
    private int Deep = -1;
    private int value = 0;
    public int findBottomLeftValue(TreeNode root) {
        value = root.val;
        findLeftValue(root,0);
        return value;
    }

    private void findLeftValue (TreeNode root,int deep) {
        if (root == null) return;
        if (root.left == null && root.right == null) {
            if (deep > Deep) {
                value = root.val;
                Deep = deep;
            }
        }
        if (root.left != null) findLeftValue(root.left,deep + 1);
        if (root.right != null) findLeftValue(root.right,deep + 1);
    }
}