【LeetCode】 112. 路径总和 递归 迭代

题目

题解

两种解法，一种是递归，一种是用栈，我想到了用栈，但是没想到要用两个栈…

递归：

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public boolean hasPathSum(TreeNode root, int sum) {
        if (root == null) return false;
        sum -= root.val;
        if (root.left == null && root.right == null) {
            if (sum == 0) return true;
        }
        return hasPathSum(root.left, sum) || hasPathSum(root.right, sum);
    }
}

迭代：

class Solution {
  public boolean hasPathSum(TreeNode root, int sum) {
    if (root == null)
      return false;

    LinkedList<TreeNode> node_stack = new LinkedList();
    LinkedList<Integer> sum_stack = new LinkedList();
    node_stack.add(root);
    sum_stack.add(sum - root.val);

    TreeNode node;
    int curr_sum;
    while ( !node_stack.isEmpty() ) {
      node = node_stack.pollLast();
      curr_sum = sum_stack.pollLast();
      if ((node.right == null) && (node.left == null) && (curr_sum == 0))
        return true;

      if (node.right != null) {
        node_stack.add(node.right);
        sum_stack.add(curr_sum - node.right.val);
      }
      if (node.left != null) {
        node_stack.add(node.left);
        sum_stack.add(curr_sum - node.left.val);
      }
    }
    return false;
  }
}