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.

解法： 深度优先遍历，时间复杂度O(n)

public boolean hasPathSum(TreeNode root, int sum) {
   return pathSum(root, 0, sum); 
}

private boolean pathSum(TreeNode root, int sum, int target) {
    if (root == null) {
        return false;
    }
    sum += root.val;
    if (root.left == null && root.right == null) {
        if (sum == target) {
            return true;
        } else {
            return false;
        }
    }
    return pathSum(root.left, sum, target) || pathSum(root.right, sum, target);
}