题目:111. 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.
1,递归
思路:根据二叉树递归的定义
public int minDepth(TreeNode root) {
if(root == null){
return 0;
}
if(root.left != null && root.right != null){
return Math.min(minDepth(root.left),minDepth(root.right)) + 1;
}
if(root.left == null && root.right == null){
return 1;
}
if(root.left != null && root.right == null){
return minDepth(root.left) + 1;
}
return minDepth(root.right) + 1;
}
2,非递归
思路:利用层次遍历, 在对每一层进行遍历的时候,第一次叶子节点出现的层数就是二叉树的最小高度
public int minDepth(TreeNode root) {
if(root == null){
return 0;
}
List<TreeNode> curLevelNodes = new ArrayList<TreeNode>();
curLevelNodes.add(root);
int level = 0;
while(!curLevelNodes.isEmpty()){
level++;
List<TreeNode> nextLevelNodes = new ArrayList<TreeNode>();
for(TreeNode node : curLevelNodes){
//按层遍历,第一个出现的叶子节点就是该二叉树最低的高度
if(node.left == null && node.right == null){
return level;
}
if(node.left != null){
nextLevelNodes.add(node.left);
}
if(node.right != null){
nextLevelNodes.add(node.right);
}
}
curLevelNodes = nextLevelNodes;
}
return level;
}
