LC.P437[路径总和III]

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 {
    int ans, target;

    public int pathSum(TreeNode root, int targetSum) {
        this.target = targetSum;
        dfs1(root);
        return ans;
    }

    private void dfs1(TreeNode node) {
        if (node == null) return;
        dfs2(node, node.val);
        dfs1(node.left);
        dfs1(node.right);
    }

    private void dfs2(TreeNode node, long val) {
        if (val == target) ++ans;
        if (node.left != null) dfs2(node.left, node.left.val + val);
        if (node.right != null) dfs2(node.right, node.right.val + val);
    }
}
  • 时间复杂度：$O(n^2)$
  • 空间复杂度：$O(n)$

方法二：前缀和+DFS+回溯

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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 {
    int ans, target;
    Map<Long, Integer> map;

    public int pathSum(TreeNode root, int targetSum) {
        if (root == null) return 0;
        this.target = targetSum;
        map = new HashMap<>();
        map.put(0L, 1);
        dfs(root, root.val);
        return ans;
    }

    private void dfs(TreeNode node, long val) {
        if (map.containsKey(val - target)) ans += map.get(val - target);
        map.merge(val, 1, Integer::sum);
        if (node.left != null) dfs(node.left, node.left.val + val);
        if (node.right != null) dfs(node.right, node.right.val + val);
        map.put(val, map.getOrDefault(val, 0) - 1);
    }
}
  • 时间复杂度：$O(n)$
  • 空间复杂度：$O(n)$