題目: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,遞歸
思路:根據(jù)二叉樹遞歸的定義
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,非遞歸
思路:利用層次遍歷, 在對每一層進行遍歷的時候,第一次葉子節(jié)點出現(xiàn)的層數(shù)就是二叉樹的最小高度
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){
//按層遍歷,第一個出現(xiàn)的葉子節(jié)點就是該二叉樹最低的高度
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;
}
