112. 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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Solution1: 

递归。没什么好分析的。

/**
 * Definition for a binary tree node.
 * 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;
        }
  if(root.left==null&&root.right==null)
  {
      if(root.val==sum)
      {
      return true;
      }
      else
      {
          return false;
      }
  }
  return hasPathSum(root.left,sum-root.val)||hasPathSum(root.right,sum-root.val);
}
}