二叉树的各种遍历【hao】

二叉树的各种遍历，包括前序，中序，后序（递归/非递归），从上往下层次遍历，从下往上层次遍历

 

思路：

前序，中序，后序的非递归遍历：基本是手动模拟递归栈调用，后序比较麻烦

正向按层遍历：利用队列，从对头出节点，然后把该节点的左孩子，右孩子加入对尾，依此类推

反向按层遍历：利用队列和栈，从对头出节点，放入栈中，然后把该节点的左孩子，右孩子加入对尾，依此类推

 

代码：

#include <iostream> #include <stack> #include <queue> #include <cstdlib> using namespace std; struct node_t { int data; node_t *l; node_t *r; node_t(int d) { data = d; l = NULL; r = NULL; } }; node_t *build_tree() { node_t *root = new node_t(0); node_t *p = root; p->l = new node_t(1); p->l->l = new node_t(3); p->l->l->r = new node_t(9); p->l->r = new node_t(4); p->l->r->l = new node_t(6); p->r = new node_t(2); p->r->l = new node_t(7); p->r->l->r = new node_t(8); return root; } /* recursive traverse */ void post_order_recu(node_t *p) { if(!p) return; post_order_recu(p->l); post_order_recu(p->r); cout<<p->data<<" "; } void in_order_recu(node_t *p) { if(!p) return; in_order_recu(p->l); cout<<p->data<<" "; in_order_recu(p->r); } void pre_order_recu(node_t *p) { if(!p) return; cout<<p->data<<" "; pre_order_recu(p->l); pre_order_recu(p->r); } /* non-recursive traverse */ void pre_order(node_t *p) { stack<node_t *> s; node_t *t = p; while(t || !s.empty()){ if(t){ cout<<t->data<<" "; s.push(t); t = t->l; }else{ t = s.top(); s.pop(); t = t->r; } } } void in_order(node_t *p) { stack<node_t *> s; node_t *t = p; while(t || !s.empty()) { if(t){ s.push(t); t = t->l; }else { t = s.top(); s.pop(); cout<<t->data<<" "; t = t->r; } } } void post_order(node_t *p) { stack<node_t *> s; node_t *t = p; node_t *tmp; while(t || !s.empty()) { if(t){ s.push(t); t = t->l; }else{ t = s.top(); /* check if right child has been visited */ if(t->r && t->r != tmp){ t = t->r; } else{ tmp = s.top(); s.pop(); cout<<tmp->data<<" "; /* we should get another node from stack */ t = NULL; } } } } /* traverse from top to bottom */ void level_order(node_t *p) { node_t *t = p; queue<node_t *> q; q.push(t); while(!q.empty()) { t = q.front(); cout<<t->data<<" "; q.pop(); if(t->l) q.push(t->l); if(t->r) q.push(t->r); } } /* traverse from bottom to top */ void reve_level_order(node_t *p) { node_t *t = p; stack<node_t *> s; queue<node_t *> q; q.push(t); while(!q.empty()){ t = q.front(); q.pop(); s.push(t); if(t->l) q.push(t->l); if(t->r) q.push(t->r); } while(!s.empty()){ cout<<s.top()->data<<" "; s.pop(); } } int main() { node_t *root = build_tree(); level_order(root); cout<<endl; post_order(root); cout<<endl; }