104. Maximum Depth of Binary Tree

Given a binary tree, find its maximum depth.

The maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.

For example:
Given binary tree [3,9,20,null,null,15,7],

3
/ \
9 20
/ \
15 7
return its depth = 3.

題意：求二叉樹最大深度。
思路：
方法1：遞歸

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public int maxDepth(TreeNode root) {
       if(root==null) return 0;
        return 1+Math.max(maxDepth(root.left),maxDepth(root.right));
    }
}

方法二：迭代 利用棧
考慮節點棧不爲空和當前節點不爲空

public static int TreeDepth(TreeNode root) {
        TreeNode node = root;
        Stack<TreeNode> nodeStack = new Stack<TreeNode>();
        Stack<Integer>  depthStack = new Stack<Integer>();
        int depth = 1;
        int max = 0;
        while(node != null || nodeStack.size()>0){
            if(node!= null){
                nodeStack.push(node);
                depthStack.push(depth);
                node = node.left;
                depth++;
            }else{
                node = nodeStack.pop();
                depth = depthStack.pop();
                if(depth > max){
                    max = depth;
                }
                node = node.right;
                depth++;
            }
        }
        return max;
    }

方法三：
BFS
遍歷每一層放進隊列

class Solution {
    public int maxDepth(TreeNode root) {
        if (root == null) return 0;

        int depth = 0;
        Queue<TreeNode> nodes = new LinkedList<>();
        nodes.offer(root);
        while (!nodes.isEmpty()) {
            int size = nodes.size();
            depth++;
            while (size-- > 0) {
                TreeNode node = nodes.poll();
                if (node.left != null) nodes.offer(node.left);
                if (node.right != null) nodes.offer(node.right);
            }
        }
        return depth;
    }
}

測試用例：

public class Solution {
    public static void main(String[] args){
        TreeNode node0 = new TreeNode(0);
        TreeNode node1 = new TreeNode(1);
        TreeNode node2 = new TreeNode(2);
        TreeNode node3 = new TreeNode(3);
        TreeNode node4 = new TreeNode(4);
        node0.left = node1;
        node0.right = node2;
        node1.left = node3;
        node3.left = node4;
        System.out.println(TreeDepth(node0));
    }
    public static int TreeDepth(TreeNode root) {
        return root!=null ? 1+Math.max(TreeDepth(root.left),TreeDepth(root.right)):0;
    }
}


class TreeNode {
    int val = 0;
    TreeNode left = null;
    TreeNode right = null;

    public TreeNode(int val) {
        this.val = val;

    }
}