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判断是否是完全二叉树

    boolean isCompleteTree(TreeNode root){if (root == null){return true;}//创建队列Queue<TreeNode> queue = new LinkedList<>();//把根放进队列里queue.offer(root);while (!queue.isEmpty()){//把出队列的放进cur里TreeNode cur = queue.poll();//当cur不等于空时，把cur的左子树和右子树放进队列if (cur != null){queue.offer(cur.left);queue.offer(cur.right);}else{//如果cur放进了null，说明要跳出队列进入判断环节break;}}while(!queue.isEmpty()){TreeNode tmp = queue.peek();//瞄一眼队列的数if (tmp == null){queue.poll();}else{//遇到不为空的说明不是完全二叉树return false;}}//来到这里说明tmp全部是空的,是完全二叉树return true;}

找出p和q的最近的公共祖先

1.root节点是p或q其中的一个，那么root就是最近的公共祖先

2.p和q分别在root的两侧，那么root是最近的公共祖先

3.p和q在root的同一侧

原理：root还是在遍历这棵树，遇到p或q就返回。

public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {if (root == null) return null;if (root == p || root == q) {return root;}TreeNode leftTree = lowestCommonAncestor(root.left, p, q);TreeNode rightTree = lowestCommonAncestor(root.left, p, q);if (leftTree != null && rightTree != null) {return root;} else if (leftTree != null) {return leftTree;} else {return rightTree;}}

还有第二种方法
大概意思就是：找p那条路径，和q那条路径出现的节点，然后放进两个栈里，保证两个栈的数相同，多的去掉，然后栈中相同的元素就是他们最近的公共祖先。

public class BinaryTree {static class TreeNode{public char val;public  TreeNode left;public  TreeNode right;public TreeNode(char val) {this.val = val;}}public TreeNode creatTree(){TreeNode A = new TreeNode('A');TreeNode B = new TreeNode('B');TreeNode C = new TreeNode('C');TreeNode D = new TreeNode('D');TreeNode E = new TreeNode('E');TreeNode F = new TreeNode('F');TreeNode G = new TreeNode('G');TreeNode H = new TreeNode('H');A.left = B;A.right = C;B.left = D;B.right = E;C.left = F;C.right = G;E.right = H;return A;}public TreeNode lowestCommonAncestor2(TreeNode root,TreeNode p,TreeNode q){if(root == null) return null;//创建两个栈Stack<TreeNode> stackP = new Stack<>();Stack<TreeNode> stackQ = new Stack<>();//两条路径getPath(root,p,stackP);getPath(root,q,stackQ);//大小int sizeP = stackP.size();int sizeQ = stackP.size();if (sizeP > sizeQ){int size = sizeP - sizeQ;while (size != 0){stackP.pop();size--;}}else {int size = sizeQ - sizeP;while (size != 0){stackQ.pop();size--;}}//两个栈元素一样多while(!stackP.isEmpty() && !stackQ.isEmpty()){if (stackP.peek() == stackQ.peek()){return stackP.peek() ;}else{stackP.pop();stackQ.pop();}}return null;}private boolean getPath(TreeNode root, TreeNode node, Stack<TreeNode> stack){if (root == null || node == null){return false;}stack.push(root);if (root == node){return true;}boolean flg1 = getPath(root.left, node, stack);if(flg1){return true;}boolean flg2 = getPath(root.right, node, stack);if(flg2){return true;}stack.pop();return false;}
}

非递归实现前序遍历

    //递归实现前序遍历void preOrder(TreeNode root){//根左右if(root == null){return;}System.out.print(root.val+" ");preOrder(root.left);preOrder(root.right);}
//非递归实现前序遍历void preOrderNor(TreeNode root) {if (root == null) {return;}Stack<TreeNode> stack = new Stack<>();TreeNode cur = root;while (cur != null || !stack.isEmpty()) {while (cur != null) {stack.push(cur);System.out.print(cur.val + " ");cur = cur.left;}TreeNode top = stack.pop();cur = top.right;//1.为空，返回；不为空创建栈，让cur=root;//当cur!=null时，把cur放进栈里，并打印cur.val；再让cur=root.left。//当cur==null时，让top=栈顶元素,然后让cur=top.right}}

非递归实现中序遍历

    //中序遍历void inOrder(TreeNode root){//左根右if(root == null){return;}inOrder(root.left);System.out.print(root.val+" ");inOrder(root.right);}//非递归中序遍历void inorderNor(TreeNode root){if (root == null){return;}Stack<TreeNode> stack = new Stack<>();TreeNode cur = root;while(cur != null || !stack.isEmpty()){while(cur != null) {stack.push(cur);cur = cur.left;}TreeNode top = stack.pop();System.out.print(top.val+" ");cur = top.right;}//不为空，创建栈，让cur=root,把cur放进栈里，然后遍历cur的左边。//直到cur遇到空，说明cur的左边遍历完了//让top=栈顶元素，并打印top的值，让cur=top.right。}

非递归实现后序遍历

    //后序遍历void postOrder(TreeNode root){//左右根if(root == null){return;}postOrder(root.left);postOrder(root.right);System.out.print(root.val+" ");}//非递归后序遍历void postOrderNor(TreeNode root){if (root == null){return;}Stack<TreeNode> stack = new Stack<>();TreeNode cur = root;TreeNode prev = null;while(cur != null || !stack.isEmpty()){while(cur != null) {stack.push(cur);cur = cur.left;}}TreeNode top = stack.peek();if (top.right == null || top.right == prev){System.out.print(top.val+" ");stack.pop();prev = top;}else{cur = top.right;}//先创建栈，让cur=root，cur不等于空或者栈不为空，当cur不等于空时，让cur入栈,然后让cur=cur.left，//直到当cur等于空时,定义prev=null；让top=瞄一眼栈顶元素，如果等于空或者top.right=prev进入循环，// 循环内打印top.val，并且出栈，然后让prev=top，否则让cur=cur.right}