题目：https://leetcode-cn.com/problems/path-sum/
答案：
public boolean hasPathSum(TreeNode root, int sum) {
if(root==null) return false;
return cal(root, sum);
}
public boolean cal(TreeNode root, int sum){
if(root.left==null){
if(root.right==null){
return sum-root.val==0;
}else{
return cal(root.right, sum-root.val);
}
}else{
if(root.right==null){
return cal(root.left, sum-root.val);
}else{
boolean left = cal(root.left, sum-root.val);
boolean right = cal(root.right, sum-root.val);
return left || right;
}
}
}