二叉树的层序遍历算法 + 打印二叉树所有最左边的元素（算法）

层序遍历

/**
* 树结构定义
*/
private static class BinaryNode<T> {
	BinaryNode(T theElement) {
		this(theElement, null, null);
	}

	BinaryNode(T theElement, BinaryNode<T> lt, BinaryNode<T> rt) {
		element = theElement;
		left = lt;
		right = rt;
	}

	T element;
	BinaryNode<T> left;
	BinaryNode<T> right;
}

//层序遍历方法
public static void levelRead(BinaryNode node) {
	if (node == null) {
		return;
	}
	//计算深度
	int depth = calcDepth(node);
	
	for (int i = 0; i < depth; i++) {
		//打印每个深度上的所有节点
		readLevel(node,i);
	}
	
	
	
	
}

private static void readLevel(BinaryNode node, int i) {
	if (node == null||i<1) {
		return;
	}
	//递归打印，如果层级为1，则打印当前节点，如果不为1，则说明还在比较高的等级，需要递归从头往下继续找当前层。
	if (i == 1) {
		System.out.print(node.element+"  ");
	}
	
	readLevel(node.left, i-1);
	readLevel(node.right, i-1);
	
	
	
	
}

private static int calcDepth(BinaryNode node) {
	if (node ==null) {
		return 0;
	}
	int ld = calcDepth(node.left);
	int rd = calcDepth(node.right);
	if (ld>rd) {
		return ld+1;
	}else{
		return rd+1;
	}
}

打印二叉树所有最左边的元素

在层序遍历的基础上，我们可以借此实现打印二叉树上所有最左边的元素，代码如下：

public static void levelReadLeft(BinaryNode node) {
	if (node == null) {
		return;
	}
	int depth = calcDepth(node);
	
	for (int i = 1; i <= depth; i++) {
		String string = readLevelLeft(node,i);
		System.out.println(string);
	}
}

private static String readLevelLeft(BinaryNode node, int i) {
	String reString = "";
	if (node == null||i<1) {
		return reString;
	}
	if (i == 1) {
		return reString + (node.element+"  ");
	}
	
	reString += readLevelLeft(node.left, i-1);
	if (reString.equals ("")) {
		reString += readLevelLeft(node.right, i-1);
	}
	return reString;
}




private static int calcDepth(BinaryNode node) {
	if (node ==null) {
		return 0;
	}
	int ld = calcDepth(node.left);
	int rd = calcDepth(node.right);
	if (ld>rd) {
		return ld+1;
	}else{
		return rd+1;
	}
}

从顶层遍历最右边节点

代码如下：

 public static void levelReadRight(BinaryNode node) {
	if (node == null) {
		return;
	}
	int depth = calcDepth(node);
	
	for (int i = 1; i <= depth; i++) {
		String string = readLevelRight(node,i);
		System.out.println(string);
	}
}

private static String readLevelRight(BinaryNode node, int i) {
	String reString = "";
	if (node == null||i<1) {
		return reString;
	}
	if (i == 1) {
		return reString + (node.element+"  ");
	}
	
	reString += readLevelRight(node.right, i-1);
	if (reString.equals ("")) {
		reString += readLevelRight(node.left, i-1);
	}
	return reString;
}




private static int calcDepth(BinaryNode node) {
	if (node ==null) {
		return 0;
	}
	int ld = calcDepth(node.left);
	int rd = calcDepth(node.right);
	if (ld>rd) {
		return ld+1;
	}else{
		return rd+1;
	}
}