Leetcode Path Sum

Given a binary tree and a sum, determine if the tree has a root-to-leaf path such that adding up all the values along the path equals the given sum.

For example:
Given the below binary tree and sum = 22,

              5
             / \
            4   8
           /   / \
          11  13  4
         /  \      \
        7    2      1

return true, as there exist a root-to-leaf path 5->4->11->2 which sum is 22.

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解题思路：

想了半天也没想明白，只好看答案，拍案叫绝。

方法一： recursion. 弱点依然是recursion. 

方法二:  use queue, 用BFS。

Add all node to a queue and store sum value of each node to another queue. When it is a leaf node, check the stored sum value.

For the tree above, the queue would be: 5 - 4 - 8 - 11 - 13 - 4 - 7 - 2 - 1. It will check node 13, 7, 2 and 1. This is a typical breadth first search(BFS) problem.

记住！！！！！ 

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 Java code:

方法一： recursion. 弱点依然是recursion. 

public boolean hasPathSum(TreeNode root, int sum) {
    if (root == null)
        return false;
    if (root.val == sum && (root.left == null && root.right == null))
        return true;
 
    return hasPathSum(root.left, sum - root.val)
            || hasPathSum(root.right, sum - root.val);
}

方法二:  use queue, 用BFS。

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public boolean hasPathSum(TreeNode root, int sum) {
        if(root == null) return false;
 
        LinkedList<TreeNode> nodes = new LinkedList<TreeNode>();
        LinkedList<Integer> values = new LinkedList<Integer>();
 
        nodes.add(root);
        values.add(root.val);
 
        while(!nodes.isEmpty()){
            TreeNode curr = nodes.poll();
            int sumValue = values.poll();
 
            if(curr.left == null && curr.right == null && sumValue==sum){
                return true;
            }
 
            if(curr.left != null){
                nodes.add(curr.left);
                values.add(sumValue+curr.left.val);
            }
 
            if(curr.right != null){
                nodes.add(curr.right);
                values.add(sumValue+curr.right.val);
            }
        }
        return false;
    }
} 

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Reference:

1. http://www.programcreek.com/2013/01/leetcode-path-sum/