Minimum Depth of Binary Tree
Given a binary tree, find its minimum depth.
The minimum depth is the number of nodes along the shortest path from the root node down to the nearest leaf node.
BFS碰到一个叶子结点就可以了。
1 class Solution { 2 public: 3 int minDepth(TreeNode *root) { 4 if (root == NULL) return NULL; 5 6 queue<TreeNode*> q; 7 q.push(root); 8 q.push(NULL); 9 int h = 1; 10 while (q.size() > 1) { 11 TreeNode* p = q.front(); 12 q.pop(); 13 14 if (p == NULL) { h++; q.push(NULL); continue;} 15 if (p->left == NULL && p->right == NULL) break; 16 if (p->left) q.push(p->left); 17 if (p->right) q.push(p->right); 18 } 19 return h; 20 } 21 22 };
这里用个NULL指针作哨兵，作为层的结束标志。所有遍历完时，q.size() == 1（q里面只有NULL一个点)。 不过这里因为只要到达叶子结点就会退出，所以不存在死循环的问题。
第三次，bugfree一遍通过。
1 class Solution { 2 public: 3 int minDepth(TreeNode *root) { 4 if (root == NULL) return 0; 5 vector<vector<TreeNode*> > layers(2); 6 int cur = 0, next = 1, layer = 1; 7 layers[cur].push_back(root); 8 while (!layers[cur].empty()) { 9 layers[next].clear(); 10 for (auto node: layers[cur]) { 11 if (node->left == NULL && node->right == NULL) return layer; 12 if (node->left) layers[next].push_back(node->left); 13 if (node->right) layers[next].push_back(node->right); 14 } 15 cur = !cur, next = !next; 16 layer++; 17 } 18 return layer; 19 } 20 };
Maximum Depth of Binary Tree
同样是用bfs好记录层数，然后bfs结束返回值就行了。
1 class Solution { 2 public: 3 int maxDepth(TreeNode *root) { 4 if (root == NULL) return 0; 5 vector<vector<TreeNode*> > layers(2); 6 int cur = 0, next = 1, layer = 0; 7 layers[cur].push_back(root); 8 while (!layers[cur].empty()) { 9 layers[next].clear(); 10 for (auto node: layers[cur]) { 11 if (node->left) layers[next].push_back(node->left); 12 if (node->right) layers[next].push_back(node->right); 13 } 14 cur = !cur, next = !next; 15 layer++; 16 } 17 return layer; 18 } 19 };