[leetcode]112. Path Sum

题目描述：

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.

递归解法

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);}

BFS

public boolean hasPathSum(TreeNode root, int sum) {        if (root == null) return false;        Queue<TreeNode> queue = new LinkedList<TreeNode>();        Queue<Integer> value = new LinkedList<Integer>();        queue.offer(root); value.offer(root.val);        while(!queue.isEmpty()){            int levelNum = queue.size();            for (int i = 0; i < levelNum; i++){                //TreeNode node = queue.peek(); int val = value.peek();                // if (queue.peek().left == null && queue.peek().right == null){                //      if (value.peek() == sum) {System.out.println(value.peek()); return true;}                // }                TreeNode node = queue.peek(); int val = value.peek();                if (node.left == null && node.right == null && val == sum) return true;                if (node.left != null) {                    value.offer(value.peek() + queue.peek().left.val);                    queue.offer(queue.peek().left);                }                if (queue.peek().right != null) {                    value.offer(value.peek() + queue.peek().right.val);                    queue.offer(queue.peek().right);                }                queue.poll();                value.poll();            }        }        return false;    }

DFS

public boolean hasPathSum(TreeNode root, int sum) {        if (root == null) return false;        //if (root.val == sum) return true;        Stack<TreeNode> stack = new Stack<>();        Stack<Integer> value = new Stack<>();        stack.push(root);value.push(root.val);        while (!stack.isEmpty()){            TreeNode node = stack.pop(); Integer val = value.pop();            if (node.left == null && node.right == null && val == sum) return true;            if (node.left != null) {                stack.push(node.left); value.push(val+ node.left.val);            }            if (node.right != null) {                stack.push(node.right); value.push(val+ node.right.val);            }            //if (stack.peek().left != null && stack.peek().right != null) stack.pop();        }        return false;    }