public class P0112PathSumBT {

public boolean hasPathSum(TreeNode root, int targetSum) {

if(root == null) return false;// 如果这棵树不存在

if(root.left == null && root.right == null) return root.val == targetSum;

boolean leftSub = hasPathSum(root.left,targetSum - root.val);

boolean rightSub = hasPathSum(root.right,targetSum - root.val);

return leftSub || rightSub;

}

boolean isValid = false;//只要有一条path符合条件就形，所以这里布尔值初始值是false

int targetSum;

public boolean hasPathSum1(TreeNode root, int targetSum) {

this.targetSum=targetSum;

sumOfTree(root, 0);

return isValid;

}

private void sumOfTree(TreeNode root, int sum) {

if(root==null) return;

sum+=root.val;

if(root.left==null&&root.right==null){

if(sum==targetSum){

isValid=true;

}

return;

}

sumOfTree(root.left, sum);

sumOfTree(root.right, sum);

}

boolean res = false;

public boolean hasPathSum2(TreeNode root, int targetSum) {

//自上而下的做法，将差额传入下一层，判断到达叶子节点时候是否是0

if(root == null) return false;

helper(root,targetSum);

return res;

}

void helper(TreeNode node, int target){

if(node == null) return;

target -= node.val;//只要node不是null，减去它的值

if (node.left == null && node.right == null && target == 0) {

res = true;

return;

}

helper(node.left,target);

helper(node.right,target);

}

}