【56】116. Populating Next Right Pointers in Each Node

/**
 * Definition for binary tree with next pointer.
 * struct TreeLinkNode {
 *  int val;
 *  TreeLinkNode *left, *right, *next;
 *  TreeLinkNode(int x) : val(x), left(NULL), right(NULL), next(NULL) {}
 * };
 */
class Solution {
public:
    void connect(TreeLinkNode *root) {
        queue<TreeLinkNode*> q;
        if(!root) return;
        q.push(root);
        q.push(NULL);//比层序遍历多了要push NULL进去
        //while(!q.empty()){
        while(true){
            //int size = q.size();
            //for(int i = 0; i < size; i++){
                TreeLinkNode* tmp = q.front();
                q.pop();
                if(tmp){
                    tmp -> next = q.front();
                    if(tmp -> left) q.push(tmp -> left);
                    if(tmp -> right) q.push(tmp -> right);
                }else{
                    if(q.size() == 0 || q.front() == NULL) return;//添加空指针NULL来达到分层的目的，使每层的最后一个节点的next可以指向NULL
                    q.push(NULL);
                }
            //}
        }
    }
};

/**
 * Definition for binary tree with next pointer.
 * struct TreeLinkNode {
 *  int val;
 *  TreeLinkNode *left, *right, *next;
 *  TreeLinkNode(int x) : val(x), left(NULL), right(NULL), next(NULL) {}
 * };
 */
class Solution {
public:
    void connect(TreeLinkNode *root) {
        queue<TreeLinkNode*> q;
        if(!root) return;
        q.push(root);
        while(!q.empty()){
            int size = q.size();
            for(int i = 0; i < size; i++){
                TreeLinkNode* tmp = q.front();
                q.pop();
                
                if(i < size - 1){//给直到每层的倒数第二个node设置next指针
                    tmp -> next = q.front();
                }
                //tmp -> next = q.front();
                
                if(tmp -> left) q.push(tmp -> left);
                if(tmp -> right) q.push(tmp -> right);
            }
        }
    }
};

/**
 * Definition for binary tree with next pointer.
 * struct TreeLinkNode {
 *  int val;
 *  TreeLinkNode *left, *right, *next;
 *  TreeLinkNode(int x) : val(x), left(NULL), right(NULL), next(NULL) {}
 * };
 */
class Solution {
public:
    void connect(TreeLinkNode *root) {
        if(!root) return;
        if(root -> left){
            root -> left -> next = root -> right;
        }
        if(root -> right){
            root -> right -> next = root -> next ? root -> next -> left : NULL;//需要判断root -> next是否为空，否则空指针是没有左指针的
        }
        connect(root -> left);
        connect(root -> right);
    }
};