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.

Note: A leaf is a node with no children.

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.

/**
 * 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;
        
        if(root.left == null  && root.right== null)
            return sum == root.val;
        
        return hasPathSum(root.left, sum - root.val) || hasPathSum(root.right,sum - root.val);
    }
    
   
}

dfs 迭代写法

/**
 * 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;
        
        Stack<TreeNode> stack = new Stack<>();
        Stack<Integer> sumStack = new Stack<>();
        
        stack.push(root);
        sumStack.push(root.val);
        
        while(!stack.isEmpty()){
            TreeNode node = stack.pop();
            
            int tmpVal = sumStack.pop();
            
            if(node.left == null && node.right == null && tmpVal == sum)
                return true;
            
            else{
                if(node.right != null){
                    stack.push(node.right);
                    sumStack.push(node.right.val + tmpVal);
                }
                
                if(node.left != null){
                    stack.push(node.left);
                    sumStack.push(node.left.val + tmpVal);
                }
            }
        }
        
        return false;
    }
}