算法：二叉树

最大深度：

解法1：<Recursive>

1 public class Solution {
2     public int maxDepth(TreeNode root) {
3         if (root == null) return 0;
4         int rst = Math.max(maxDepth(root.left), maxDepth(root.right));
5         return rst + 1;
6     }
7 }

解法2：<Iterative>（层序遍历思想: queue+size+cnt;）

 1 public int maxDepth(TreeNode root) {
 2     if(root == null) {
 3         return 0;
 4     }
 5     Queue<TreeNode> queue = new LinkedList<>();
 6     queue.offer(root);
 7     int count = 0;
 8     while(!queue.isEmpty()) {
 9         int size = queue.size();
10         while(size-- > 0) {
11             TreeNode node = queue.poll();
12             if(node.left != null) {
13                 queue.offer(node.left);
14             }
15             if(node.right != null) {
16                 queue.offer(node.right);
17             }
18         }
19         count++;
20     }
21     return count;
22 }

2. Minimum Depth of Binary Tree：https://leetcode.com/problems/minimum-depth-of-binary-tree/

最小深度：

解法1：<Recursice>（左节点为null返回右边的min+1；右节点为null返回左边的min+1；都不为null则返回Math.min()+1;）

1 public class Solution {
2     public int minDepth(TreeNode root) {
3         if (root == null) return 0;
4         if (root.left == null) return minDepth(root.right) + 1;
5         if (root.right == null) return minDepth(root.left) + 1;
6         int rst = Math.min(minDepth(root.left), minDepth(root.right));
7         return rst + 1;
8     }
9 }

解法2：<Iterative>（层序遍历思想；找到第一个叶节点就返回）

public class Solution {
    public int minDepth(TreeNode root) {
        if (root == null) return 0;
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        int count = 1;
        while (!queue.isEmpty()) {
            int size = queue.size();
            for (int i = 0; i < size; i++) {
                TreeNode node = queue.poll();
                if (node.left == null && node.right == null) return count;
                if (node.left != null) queue.offer(node.left);
                if (node.right != null) queue.offer(node.right);
            }
            count++;
        }
        return count;
    }
}

3. Binary Tree Inorder Traversal：https://leetcode.com/problems/binary-tree-inorder-traversal/

中序遍历：

解法1：<Recursive>(addAll)

public class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> rst = new ArrayList<>();
        if (root == null) return rst;
        rst.addAll(inorderTraversal(root.left));
        rst.add(root.val);
        rst.addAll(inorderTraversal(root.right));
        return rst;
    }
}

解法2：<Iterative>