leetcode 404 Sum of Left Leaves
z
 2019-04-17 09:49:51  

Find the sum of all left leaves in a given binary tree.

Example:

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    3
   / \
  9  20
    /  \
   15   7

There are two left leaves in the binary tree, with values 9 and 15 respectively. Return 24.
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/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public int sumOfLeftLeaves(TreeNode root) {
        if (root == null)
            return 0;
        return helper(root, false);
    }
    
    public int helper(TreeNode root, boolean isLeft) {
        if (root == null)
            return 0;
        if (isLeft && root.left == null && root.right == null) 
            return root.val;
        return helper(root.left, true) + helper(root.right, false);
    }
}
  • Using stack
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public int sumOfLeftLeaves(TreeNode<Integer> root) {
    int res = 0;
    if (root == null) return res;
    Stack<TreeNode<Integer>> stack = new Stack<>();
    while (root != null || !stack.isEmpty()) {
        while (root != null) {
            stack.push(root);
            root = root.right;
        }
        root = stack.pop();
        root = root.left;
        if (root != null && root.left == null && root.right == null) res += root.val;
    }
    return res;
}