二叉树的深度遍历和广度遍历

深度遍历，即尽可能往树的纵深的方向搜索，所以优先深入遍历树的左节点，直至叶子节点，再往回遍历已遍历的节点的右节点。使用栈来实现

package com.practice;

import java.util.LinkedList;

public class DepthFirstSearch {
	
	public static TreeNode createTree(int[] arr, int i) {
		TreeNode root = null;
		if(i<arr.length) {
			root = new TreeNode(arr[i]);
			root.left = createTree(arr, 2*i+1);
			root.right = createTree(arr, 2*i+2);
		}
		return root;
 		
	}
	public static void search(int[] arr) {
		TreeNode root = createTree(arr, 0);
		if(root==null)	return;
		LinkedList<TreeNode> list = new LinkedList<TreeNode>();
		list.add(root);
		while(!list.isEmpty()) {
			TreeNode node = list.pop();
			System.out.print(node.val+",");
			if(node.right!=null)	list.push(node.right);
			if(node.left!=null)		list.push(node.left);
		}
	}
	
	public static void main(String[] args) {
		int[] arr = { 2, 7, 8, 3, 1, 6, 9, 0, 5, 4 };
		search(arr);
	}
}

广度优先遍历，即层序遍历，优先将根节点遍历完，再遍历根节点的子节点。使用队列来实现

package com.practice;

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

public class WidthFirstSearch {
		
	public static TreeNode createTree(int[] arr, int i) {
		TreeNode root = null;
		if(i<arr.length) {
			root = new TreeNode(arr[i]);
			root.left = createTree(arr, 2*i+1);
			root.right = createTree(arr, 2*i+2);
		}
		return root;
	}
	
	public static void search(int[] arr) {
		TreeNode root = createTree(arr, 0);
		if(root==null)	return;
		Queue<TreeNode> queue = new LinkedList<TreeNode>();
		queue.add(root);
		while(!queue.isEmpty()) {
			int size = queue.size();
			for(int i=0;i<size;i++) {
				TreeNode node = queue.poll();
				System.out.print(node.val+",");
				if(node.left!=null) 	queue.add(node.left);
				if(node.right!=null)	queue.add(node.right);
			}
		}
	}
	public static void main(String[] args) {
		int[] arr = { 2, 7, 8, 3, 1, 6, 9, 0, 5, 4 };
		search(arr);
	}	
}