6.14找树左下角值——【LeetCode】

import java.util.LinkedList;
import java.util.Queue;
import java.util.Stack;

/**
 * @auther DNY
 * @create 2022-02-17 19:31
 */
public class test {

    //递归法：
    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){//这里的判断很重要，下面的向左递归查找 回溯结束之后  再向右递归查找 可能会有一段deep < Deep的情况存在
                value = root.val;
                Deep = deep;
            }
        }
        if(root.left != null){
            findLeftValue(root.left, deep + 1);
        }
        if(root.right != null){
            findLeftValue(root.right, deep + 1);
        }
    }

    //迭代法
    public int findBottomLeftValue1(TreeNode root) {
        Queue<TreeNode> queue = new LinkedList<>();//临时记录每一行的结点
        queue.offer(root);
        int res = 0;
        while(!queue.isEmpty()){
            int size = queue.size();
            for(int i = 0; i < size;i++){
                TreeNode poll = queue.poll();
                if(i == 0){//即res记录每一行最左边的值
                    res = poll.val;
                }
                //提前将下一行的结点添加到queue中
                if(poll.left != null){
                    queue.offer(poll.left);
                }
                if(poll.right != null){
                    queue.offer(poll.right);
                }
            }
        }
        return res;
    }
}

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;
    }
}