111. Minimum Depth of Binary Tree
Tags: ‘Tree’, ‘Depth-first Search’, ‘Breadth-first Search’
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.
Note: A leaf is a node with no children.
Example:
Given binary tree [3,9,20,null,null,15,7],
3
/ \
9 20
/ \
15 7
return its minimum depth = 2.
Solution
方法1： 简单的DFS，需要global记录min, 不太好
private int min = Integer.MAX_VALUE;
public int minDepth(TreeNode root) {
if (root == null) return 0;
helper(root, 1);
return min;
}
private void helper(TreeNode root, int depth) {
if (root == null) {
return;
}
if (root.left == null && root.right == null) {
min = Math.min(min, depth);
}
if (root.left != null) {
helper(root.left, depth + 1);
}
if (root.right != null) {
helper(root.right, depth + 1);
}
}
方法2： DFS，不需要global
public int minDepth(TreeNode root) {
if (root == null) return 0;
if (root.left == null) {
return minDepth(root.right) + 1;
} else if (root.right == null) {
return minDepth(root.left) + 1;
}
return Math.min(minDepth(root.left), minDepth(root.right)) + 1;
}