迭代----- 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.

Have you met this question in a real interview? Yes
Example
Given a binary tree as follow:

1
/ \
2 3
/ \
4 5
The maximum depth is 3.

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解法0:

/**
 * Definition of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 */
public class Solution {
    public int maxDepth(TreeNode root) {
        if (root == null) {
            return 0;
        }

        int left = maxDepth(root.left);
        int right = maxDepth(root.right);
        return Math.max(left, right) + 1;
    }
}

解法1:

/**
 * Definition of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 */
public class Solution {
    /**
     * @param root: The root of binary tree.
     * @return: An integer.
     */
    public int maxDepth(TreeNode root) {
    if(root == null)
        return 0;

    int depth = 0;
    LinkedList<TreeNode> queue = new LinkedList<TreeNode>();
    queue.add(root);
    int curNum = 1; //num of nodes left in current level
    int nextNum = 0; //num of nodes in next level
    while(!queue.isEmpty()){
        TreeNode n = queue.poll();
        curNum--;
        if(n.left!=null){
            queue.add(n.left);
            nextNum++;
        }
        if(n.right!=null){
            queue.add(n.right);
            nextNum++;
        }
        if(curNum == 0){
            curNum = nextNum;
            nextNum = 0;
            depth++;
        }
    }
    return depth;
}
}