Offer.P34[二叉树中和为某一值的路径]

方法一：DFS

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/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    List<List<Integer>> ans = new ArrayList<>();
    List<Integer> path = new ArrayList<>();
    int target;

    public List<List<Integer>> pathSum(TreeNode root, int target) {
        this.target = target;
        dfs(root, 0);
        return ans;
    }

    private void dfs(TreeNode root, int sum) {
        if (root == null) return;
        sum += root.val;
        path.add(root.val);
        if (sum == target && root.left == null && root.right == null) {
            ans.add(new ArrayList<>(path));
        }
        dfs(root.left, sum);
        dfs(root.right, sum);
        path.remove(path.size() - 1);
    }
}
  • 时间复杂度：$O(n)$
  • 空间复杂度：$O(n)$

方法二：DFS

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/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    
    List<List<Integer>> ans = new ArrayList<>();
    List<Integer> path = new ArrayList<>();

    public List<List<Integer>> pathSum(TreeNode root, int target) {
        dfs(root, target);
        return ans;
    }

    private void dfs(TreeNode root, int target) {
        if (root == null) return;
        target -= root.val;
        path.add(root.val);
        if (target == 0 && root.left == null && root.right == null) {
            ans.add(new ArrayList<>(path));
        }
        dfs(root.left, target);
        dfs(root.right, target);
        path.remove(path.size() - 1);
    }
}
  • 时间复杂度：$O(n)$
  • 空间复杂度：$O(n)$